Homoallyhc alcohols preparation

Barrett and Lebold used the Brown asymmetric crotylation to prepare the homoallyhc alcohol 22 in the total synthesis of nikkomycin B 21, a natural product that exhibits fungicidal, insecticidal, and acaricidal activities14 (Scheme 3.1k). 

Asymmetric allyation of carbonyl compounds to prepare optically active secondary homoallyhc alcohols is a useful synthetic method since the products are easily transformed into optically active 3-hydroxy carbonyl compounds and various other chiral compounds (Scheme 1). Numerous successful means of the reaction using a stoichiometric amount of chiral Lewis acids or chiral allylmetal reagents have been developed and applied to organic synthesis however, there are few methods available for a catalytic process. Several reviews of asymmetric allylation have been pubHshed [ 1,2,3,4,5] and the most recent [5] describes the work up to 1995. This chapter is focussed on enantioselective allylation of carbonyl compounds with allylmetals under the influence of a catalytic amount of chiral Lewis acids or chiral Lewis bases. Compounds 1 to 19 [6,7,8,9,10,11,12,... 

If the carbanion of (1) is allowed to react with an aldehyde or ketone, 3-hydroxy aldehydes can be prepared. Since 3-hydroxy aldehydes are usually labile, it is advantageous to trap the adduct (a) with chloromethyl methyl ether as shown for (5). The protected hydroxy aldehydes (6) so obtained can be converted into homoallyhc alcohols by a Wittig reaction. 

In a separate investigation by Porco, Jr. and coworkers, the isatin derivative 134 (R = H) was converted to spirooxindole pyran 141 in 13 1 dr and 99% ee by means of a Prins-type cyclization involving homoaUyhc alcohol 140 [79]. The diastereos-electivity of the transformation has been proposed to arise from a preferred chairlike transition state with the benzenoid ring of oxindole in a pseudoequatorial orientation. Spirooxindole oxepenes also were prepared via diastereoselective spiro-annulation of isatins with bis-homoallyhc alcohols. 

RajanBabu et aL have described an addition-elimination sequence employing vinyl stannanes for the preparation of homoallyhc alcohols containing 5-membered rings [104]. 

A similar allyl [91] or propargyl [92] Reformatsky reagent has been used to prepare fluonnated homoallyhc or homopropargyhc alcohols, respectively [91,92] (equations 60 and 61)... 

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