Homopropargylic alcohols, asymmetric synthesis

In 1993, Hayashi and co-workers reported a catalytic asymmetric synthesis of alle-nylboranes 256 by palladium-catalyzed hydroboration of conjugated enynes 253 (Scheme 4.66) [105]. Reaction of but-l-en-3-ynes 253 with catecholborane 254 in the presence of a catalyst, prepared from Pd2(dba)3 CHC13 (1 mol%) and a chiral mono-dentate phosphine ligand (S)-MeO-MOP 255 (1 mol%), gave an allenylborane 256. The ee of 256 was determined by the reaction with benzaldehyde affording the corresponding optically active homopropargyl alcohols 257 with up to 61% ee (syn anti= 1 1—3 1). 

Scheme 1.3.17 Asymmetric synthesis of homopropargyl alcohols via a-elimination of alkylidene aminosulfoxonium ylides.

The asymmetric approaches include the preparation of the configurationally stable chiral aUenyltin starting from enantio-enriched propargyUc precursors. When submitted to transmetaUation with Sn, Bi or In Lewis acids before addition to the aldehyde, the homopropargyl alcohol is obtained in a 95 5 anti/syn ratio and in a 90% ee . On the other hand, the use of a chiral aUenyltin reagent, without prior transmetaUation, gives the syn adduct selectively (95 5) (equation 12) . However, the use of chiral aUenyltin and chiral aldehydes may lead to the same match/mismatch effect that was observed with allyltins . Both approaches were applied to the synthesis of macrolides subunits ° i . 

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