1- Alkenyl carbamates, asymmetric

It has been demonstrated by Pancrazi, Ardisson and coworkers that an efficient kinetic resolution takes place when an excess (2 equivalents) of the racemic titanated alkenyl carbamate rac-334a (R = Me) is allowed to react with the enantiopure )-hydroxyaldehyde 341 or alternatively the corresponding y-lactol 340, since the mismatched pair contributes to a lower extent to the product ratio (equation 91) . Under best conditions, the ratio of the enantiomerically pure diastereomers 3,4-anti-4,5-syn (342) and 3,4-anti-4,5-anti (343) is close to 14 1. Surprisingly, approximately 9% of the iyw,iyw-diastereomer 344 resulted when the starting (ii)-crotyl carbamate was contaminated by the (Z)-isomer. The reasons which apply here are unknown. Extra base has to be used in order to neutrafize the free hydroxy group. The pure awft, awfi-product 345 was obtained with 85% yield from the reaction of the (W-oxy-substituted titanate rac-334b and lactol 340. 345 is an intermediate in the asymmetric synthesis of tylosine . ... 

Some alkenyl carbamates leading to configurationally labile lithium intermediates could be subjected to asymmetric homoaldol reaction with less efficiency (Scheme 6) these reactions have not been optimized yet Azs... 

On the basis that a wide variety of (S)-configurated (a-carbamoyloxy)alkyllithium derivatives are accessible by (—)-sparteine-mediated deprotonation30, Hoppe and coworkers have described the synthesis of enantiomerically and diastereomerically pure cyclopen-tanols 38 by asymmetric cyclocarbolithiation reaction of 5-alkenyl carbamates like 36. Its deprotonation with s-BuLi/(—)-sparteine gives a chiral organolithium which cyclizes to benzyllithium 37 via 5-exo-trig and again with retention of configuration at the carbanionic... 

Total stereoselectivity was also observed on cyclization of /V -alkenyl carbamates with phenyl-selenenyl chloride to give TV-substituted piperidines 5. Due to the sense of the 1,3-asymmetric induction, cis-2,6-piperidine derivatives are obtained exclusively, their stereochemistry was assigned on the basis of H NMR107. 

It is especially remarkable that optically active homoaldoi adducts can be obtained when enantiomeri-cally pure 2-alkenyl carbamates (47 R = alkyl) are employed. Apparently the deprotonation occurs with retention of configuration and leads to configurationally stable liAium derivatives, which, after metal exchange with Ti(OPr )4, again with retention, add to aldehydes with efficient 1,3-chirality transfer coupled with enantiofacial differentiation at the carbonyl group, indicating a rigid six-membered transition state. Recently even an asymmetric homoaldoi reaction by enantioselective lithiation of prochiral primary alkenyl carbamates in the presence of (-)-sparteine was reported. ... 

The first asymmetric lithium-ene reaction, based on the enantioselective (-)-sparteine-induced deprotonation of 2-alkenyl carbamates, provides a route to enan-tiomerically enriched carbocycles. " ... 

As part of a review on the use of metallated 2-alkenyl carbamates as chiral homoenolate reagents for asymmetric synthesis, the preparation of the a-D-allo cofigurated methyl-branched 3,6-dideoxy compound (56) as well as the fi-D-talo derivative (57) have been reported starting from R-benzyl lactaldehyde. For the preparation of the a-L-allo and the P-L-w/o compounds see Vol. 21, p.l44. 

Butyllithium is also used for the stereoselective a-lithiation of chiral sulfonyl compounds/ chiral 2-alkenyl carbamates, various heterocyclic amine derivatives with chiral auxiliaries appended on the nitrogen (e.g. oxazoline or formamidine groups) (eq 10)7 and chiral oxazolidinones derived from ben-zylamines (eq 11). These elegant reactions have been applied to the asymmetric syntheses of a number of natural products. ... 

Alkyl- or 3-aryl-2,4-oxazolidinediones via photochemical cyclization, ° organonickel-mediated carbonylation, ° cyclization of A-alkenyl-a-acet-amides, ° carboxylation and cyclization of 2-propynamides, °" cyclization of (9-carbamates of a-hydroxy acetic acids and esters,cyclization of a-hydroxy acetamides,and catalytic asymmetric dihydroxylation (ADH) of A-alkenoyl-2-oxazolidinones. ... 

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