Stereoselective synthesis alkanes

At first glance, the requirement for the use of already functionalized alkanes seems to limit the applicability of the directed C-H activation approach. However, the utility of such a method is obvious for chemists familiar with the use of auxiliary groups during stereoselective synthesis (neighboring group participation was also applied in the total synthesis shown in Scheme 6). 

Pandey, G., Devi Reddy, G., and Chakrabarti, D. (1996) Stereoselectivity in the photoinduced electron transfer (PET) promoted intramolecular cyclisations of l-alkenyl-2-silyl-piperidines and -pyrrolidines rapid construction of 1-azabicyclo [m.n.o] alkanes and stereoselective synthesis of ( )-isoretronecanol and ( )-epilupinine. Journal of the Chemical Society, Perkin Transactions 1, 219-224. 

S.M. Weinreb and co-workers were surprised to find that the convergent stereoselective synthesis of marine alkaloid lepadiformine resulted in a product that gave a totally different NMR spectra than the natural product. This finding led to the revision of the proposed structure of lepadiformine. In the final stages of the synthesis, they exposed a tricyclic piperidone intermediate to Ciemmensen conditions to remove the ketone functionality. Under these conditions the otherwise minor elimination product (alkene) was formed predominantly however, it was possible to hydrogenate the double bond to give the desired alkane. 

Stereoselective synthesis of aza- and diazabicyclo[X.Y.0]alkane dipeptide mimetics 05S1031. 

Stereoselective Alkene Synthesis via 1-Chloro-1-[(dimethyl)-phenylsilyl]-alkanes and a-(Dimethyl)phenylsilyl Ketones. 

In agreement with Schemes 2.211b and 2.211c, intramolecular cycloadditions of nitrones to 5-allyl- (Scheme 2.225) or 5-homoallylproline (Scheme 2.226), are fully regio- and stereoselective. These reactions are the key steps in the synthesis of functionalized azaoxobicyclo[X.3.0] alkane amino acids, mimics of a homoSer-Pro dipeptide (721). 

The stereochemical aspect of these cyclizations has been investigated. They have been found to be non-stereoselectives.342 On the contrary, diastereoselective 1-azabicyclo-alkanes have been prepared in the same way, the stereochemistry being dependent upon the size of the ring formed, namely, 1,5-cis and 1,6-trans.343 This methodology has been applied to the synthesis of ( )-isoretronecanol, ( )-epilupinine344 and retronecanol.345... 

By this method (Z)-monounsaturated fatty acids and esters could be obtained with an ( )-isomer content of less than 10% this stereoselectivity being however inferior to that of the commonly used acetylenic approach 55,56). However, the salt-free techniques used today in Wittig reactions allow (Z)-alkenoic acids to be synthesized with less than 2% of the ( )-isomers. Thus, Bestmann et al. prepared methyl and ethyl esters of (Z)-4,5,6,7,8,9,ll- and 13-alkenoic acids of different chain lengths 35,57 62), which served as intermediates in the synthesis of insect pheromones, both by reaction of co-alkoxycarbonyl-substituted alkyl-triphenyl-phosphonium salts with simple alkanals and of co-formylalkanoic esters with alkylidenephosphoranes. As the starting material for the synthesis of -substituted alkyl-phosphonium salts co-chloro- and -bromocarboxylic esters were used. The corresponding -substituted aldehydes can usually be obtained by ozone cleavage of suitable olefin derivatives or by oxidation of alkohols 57,58). 

STEREOSELECTIVE ALKENE SYNTHESIS via t-CHLORO-1-[(DIMETHYL)PHENYLSILYL] ALKANES and a-(DIMETHYL)PHENYLSILYL KETONES 6-METHYL-6-DODECENE... 

Pandey, G., Laha, J.K., and Lakshmaiah, G. 2002. Stereoselective construction of X-azabicyclo[ i.2.1]alkanes by [3+2]-cycloaddition of non-stabilized cyclic azomethine ylides synthesis of enantiopure constrained amino acids and formal total synthesis of optically active epibatidine. Tetrahedron 58, 3525-3534. 

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