Enantioselective Michael Addition

Coming on the heels of the very nice combined computational/experimental study of the enantioselective Strecker reaction by Jacobsen , there’s this JACS communication that really disappoints in its use of computational chemistry. Cobb uses yet another chiral thiourea to produce the enantioselective intramolecular Michael addition of nitronoates (Reaction1).¹ The reaction goes with excellent diastereoselectivity and eneatioselectivity, and can even be done with a substrate to produce three chiral centers. This is very nice synthetic chemistry.

Reaction 1. dr >19:1 ee 95%

The lack of reactivity of the Z ester suggested that the thiourea must associate with both the nitro group and the ester carbonyl. The authors provide a B3LYP/3-21G complex of thiourea with a simple nitroester (once again without providing coordinates in the supporting materials!) to demonstrate this sort of association. But this single structure, at this very low computational level, with these simplified reagents, and lacking solvent (see Rzepa’s comment) really makes one wonder just what value this computation provides. It also goes to demonstrate just how much effort Jacobsen went through to provide substantive computational support for his proposed mechanism of action.

References

(1) Nodes, W. J.; Nutt, D. R.; Chippindale, A. M.; Cobb, A. J. A., "Enantioselective Intramolecular Michael Addition of Nitronates onto Conjugated Esters: Access to Cyclic γ-Amino Acids with up to Three Stereocenters," J. Am. Chem. Soc. 2009, 131, 16016-16017, DOI: 10.1021/ja9070915

Novel cyclophanes: Out-of-Plane Bending and Aromaticity

The novel cyclophanes 1 and 2 have now been synthesized.¹ An interesting question is whether the bent pyrenes portion of the two molecules remains aromatic. The bending angles is 93.8° in 1and 95.8° in 2. This distortion is readily apparent in Figure 1, which presents their B3LYP/6-311G(d,p) optimized geometries. NICS computations were used to assess the aromaticity of the pyrene portion. The central rings of pyrene have NICS(0) = -4.4 ppm. The corresponding values in1 and 2 are -4.5 ppm. The apical rings of pyrene have NICS(0)= -11.9 ppm, while the value is -11.1 ppm in 1 and -11.0 ppm in 2. These calculations indicate that the molecule retains much of the aromaticity of the parent pyrene despite the significant out-of-plane distortions.

Figure 1. B3LYP/6-311G(d,p) optimized geometries of 1 and 2.¹

1

2

References

(1) Zhang, B.; Manning, G. P.; Dobrowolski, M. A.; Cyranski, M. K.; Bodwell, G. J., "Nonplanar Aromatic Compounds. 9. Synthesis, Structure, and Aromaticity of 1:2,13:14-Dibenzo[2]paracyclo[2](2,7)-pyrenophane-1,13-diene," Org. Lett., 2008, 10, 273-276, DOI: 10.1021/ol702703b.

Do You Remember Darzen condensation Reaction

The Darzens reaction (also known as the Darzens condensation or glycidic ester condensation) is the chemical reaction of a ketone or aldehyde with an α-haloester in the presence of base to form an α,β-epoxy ester, also called a "glycidic ester".^[1][2] This reaction was discovered by the organic chemist Auguste George Darzens in 1904.^[3]

The reaction process begins when a strong base is used to form a carbanion at the halogenated position. Because of the ester, this carbanion is a resonance-stabilized enolate, which makes it relatively easy to form. This nucleophilic structure attacks another carbonyl component, forming a new carbon–carbon bond. These first two steps are similar to a base-catalyzed aldol reaction. The oxygen anion in this aldol-like product then does an intramolecular S_N2 attack on the formerly-nucleophilic halide-bearing position, displacing the halide to form an epoxide.^[4] This reaction sequence is thus a condensation reaction, since there is a net loss of HCl when the two reactant molecules join.Reaction mechanism

The primary role of the ester is to enable the initial deprotonation to occur, and other carbonyl functional groups can be used instead. If the starting material is an α-halo amide, the product is an α,β-epoxy amide.^[5] If an α-halo ketone is used, the product is an α,β-epoxy ketone.^[4]
Any sufficiently strong base can be used for the initial deprotonation. However, if the starting material is an ester, the alkoxide corresponding to the ester side-chain is commonly in order to prevent complications due to potential acyl exchange side reactions.

Stereochemistry

Depending on the specific structures involved, the epoxide may exist in cis and trans forms. A specific reaction may give only cis, only trans, or a mixture of the two. The specific stereochemical outcome of the reaction is affected by several aspects of the intermediate steps in the sequence.
The initial stereochemistry of the reaction sequence is established in the step where the carbanion attacks the carbonyl. Two sp³ (tetrahedral) carbons are created at this stage, which allows two different diastereomeric possibilities of the halohydrin intermediate. The most likely result is due to chemical kinetics: whichever product is easier and faster to form will be the major product of this reaction. The subsequent S_N2 reaction step proceeds with stereochemical inversion, so the cis or trans form of the epoxide is controlled by the kinetics of an intermediate step. Alternately, the halohydrin can epimerize due to the basic nature of the reaction conditions prior to the S_N2 reaction. In this case, the initially formed diastereomer can convert to a different one. This is an equilibrium process, so the cis or trans form of the epoxide is controlled by chemical thermodynamics--the product resulting from the more stable diastereomer, regardless of which one was the kinetic result.^[5]

Alternative reactions

Glycidic esters can also be obtained via nucleophilic epoxidation of an α,β-unsaturated ester, but that approach requires synthesis of the alkene substrate first whereas the Darzens condensation allows formation of the carbon–carbon connectivity and epoxide ring in a single reaction.

Subsequent reactions

The product of the Darzens reaction can be reacted further to form various types of compounds. Hydrolysis of the ester can lead to decarboxylation, which triggers a rearrangement of the epoxide into a carbonyl (4). Alternately, other epoxide rearrangements can be induced to form other structures.

References

1.  Darzens, G. (1905). Compt. Rend. 141: 766.
2.  Darzens, G. (1906). Compt. Rend. 142: 214.
3.  Darzens, G. (1904). Compt. Rend. 139: 1214.
4. Jie Jack Li (2006). "Darzens glycidic ester condensation". Name Reactions (3rd. expanded ed.). Springer-Verlag. pp. 183–184. doi:10.1007/3-540-30031-7.
5. Tung, C. C.; Speziale, A. J.; Frazier, H. W. (1963). "The Darzens Condensation. II. Reaction of Chloroacetamides with Aromatic Aldehydes". The Journal of Organic Chemistry 28 (6): 1514. doi:10.1021/jo01041a018. edit

Review articles

• Newman, M. S.; Magerlein, B. J. (1949). Org. React. 5: 413.
• Ballester, M. (1955). "Mechanisms of The Darzens and Related Condensations Manuel Ballester". Chem. Rev. 55 (2): 283. doi:10.1021/cr50002a002.
• Rosen, T. (1991). Comp. Org. Syn. 2: 409–439.

Enantioselective Carbene-Catalyzed Annulations

 

2-benzoxopinones were synthesized by a dual activation strategy, involving N-heterocyclic carbene catalysis and a second Lewis base
Read more


http://www.chemistryviews.org/details/news/5017851/Enantioselective_Carbene-Catalyzed_Annulations.html

Camphor and Rivanol (Ethacridine Lactate) - An interesting reaction

Camphor and Rivanol (Ethacridine Lactate) - An interesting reaction

http://dailychem.blogspot.in/2011/11/camphor-and-rivanol-ethacridine-lactate.html

Two days ago, while trying to clean a pimple with an antiseptic camphor solution, I accidentally used ethacridine lactate (Rivanol) instead. After realizing this, instead of first cleaning the rivanol stain and then applying the camphor, I directly cleaned it with camphor using a cotton pad. Suddenly a new white layer formed at the contact surface between the two layers of substance. The occurrence of a chemical reaction was obvious, and after repeating the experiment in lab conditions, I tried to understand the formation of this new product.

Camphor structure

Rivanol structure  READ ALL THIS AT http://dailychem.blogspot.in/2011/11/camphor-and-rivanol-ethacridine-lactate.html

Quindine catalyzed synthesis of spirooxindoles through sequential Michael-Henry reactions.

A novel organocatalytic strategy for the synthesis of highly substituted spirocyclopentaneoxindoles was developed employing simple nitrostyrenes and 3-substituted oxindoles as starting materials. Michael–Henry cascade reactions, enabled through cinchona alkaloid organocatalysis, provided products in high yield and excellent enantioselectivity in a single step.

Spirooxindoles in one step

 DOI: 10.1021/ol300441z

http://pubs.acs.org/doi/abs/10.1021/ol300441z

If you just leaf through an organic chemistry journal, from JOC to Organic Letters, from EJOC to OBC, there is a huge number of papers describing synthesis of oxindoles! Indeed, the interest in the oxindoles scaffold rapidly increased in the last years and several methodologies have been published for the asymmetric synthesis of 3,3’-disubstituted oxindoles.

Cascade organacatalysis is a powerful tool for the for the asymmetric synthesis of very complex structures containing several stereogenic centers and oxindoles are optimal templates for the applications of these wonderful catalytic methodologies. For example, prof. Paolo Melchiorre, one the most expert in the organocatalysis field, published several works in the application of organocascade catalysis for the synthesis of quaternary oxindoles.

In this communication, Carlos Barbas, another pioneer of organocatalysis, reported a quindine-catalyzed synthesis of spirooxindoles through sequential Michael-Henry reactions.

They first performed a preliminary methodological study aimed at finding the most efficient catalyst, which turned out to be quindine derivative depicted below.

Subsequently the scope of the reaction has been expanded, by employing a wide range of substituted oxindoles, as well as different nitrostyrens. Performing the reaction at 0°C in CH2Cl2 as solvent in presence of quinidine derivative 1 permitted the isolation of the products in very high yield and optimal enantiomeric excess and diastereoisomeric ratio.

It is noteworthy to mention that this simple methodology permits the straightforward construction of the spirooxindole moiety containing four stereogenic centers,, with full control of the stereochemical outcome. This is another demonstration of the powerful of organocatalysis. 

http://pubs.acs.org/doi/abs/10.1021/ol300441z

Assembly of Spirooxindole Derivatives Containing Four Consecutive Stereocenters via Organocatalytic Michael–Henry Cascade Reactions

Klaus Albertshofer , Bin Tan , and Carlos F. Barbas, III*

The Skaggs Institute for Chemical Biology and the Departments of Chemistry and Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, United States

Org. Lett., 2012, 14 (7), pp 1834–1837

DOI: 10.1021/ol300441

Efficient Odorant Synthesis

Efficient Odorant Synthesis

2-Nonen-4-olide occurs naturally in the volatile components of various foods and plant extracts, such as stewed beef/vegetable gravy,  sunflower oil,  Chinese white salted noodles watermelon, potato crisps,  lard cured with spices and aromatic herbs,  mushrooms, French fries, and flue-cured tobacco. It has an oily, coconut-like, and rancid odour. A simple, efficient and low cost synthesis of this odorant is reported in the August issue of Journal of Chemical Research.

http://www.sciencereviews2000.co.uk/blog/view/journal-of-chemical-research/58/efficient-odorant-synthesis/628