Ketones, chiral -acetoxy

Furthermore, yeast treatment of the a-acetoxy ketone 53, bearing two oxygen substituents ( ) at a and affords the carbinol 54 in 20 % yield, somewhat less than 10 % of the (2R) diastereoisomer, and 70 % of recovered starting material. From the carbinol 54, crystalline 55 is obtained, which may be converted through suitable manipulation of the protecting groups and ozonolysis, into A-deoxy-D-talo-hexose 56. The minor diastereoisomer similarly affords A-deoxy-D-xylo-hexose 57. Thus, in the yeast treatment of 53, as the results of the enzymic kinetic resolution, the (2S, 4S, 5R) diol 55, a carbohydrate-like chiral synthon, is accessible out of eight possible isomers. 

Enders and Bhushan reported the preparation of a-benzyloxy aldehydes and a-acetoxy ketones 6 of high enantiomeric purity and in good overall yield by oxygenation of the azaenolates of chiral hydrazone 4 with 3-phenyl-2-(phenylsulfonyl)oxaziridine 282. The chiral auxiliary was removed without racemization by ozonolysis of the a-hydroxy hydrazone 5 at — 78 °C. 

Enders and Bhushan described the preparation of a-acetoxy ketones of high enantiomeric purity and good overall yield by hydroxylation of the azaenolates, derived from chiral SAMP-hydrazones (149), with (63a) as the key step <88TL2437>. The chiral auxiliary in (150) can be removed, without racemization, by ozonolysis of the a-hydroxy hydrazone at — 78 °C affording the a-hydroxy ketones which were isolated as the acetates (151) (Scheme 27). Protected a-hydroxy aldehydes, difficult to prepare in other ways, are available in high enantiomeric purity in a similar manner. 

Alkylations and aldol condensations of aldehydes and ketones with enolates of chiral dioxolanes proceed generally with high di-astereoselection, e.g. eqs 26 and 27. The magnesium eno-late of 5-(+)-2-acetoxy-1,1,2-tripheny lethanol generated hy transmetalation with Magnesium Bromide has enjoyed considerable success in aldol condensations, e.g. eq 28. ... 

Enzymatic resolution of 1,4-diols with CALB in combination with a hydrogen transfer process catalyzed by Shvo s complex 1 led to an efficient DYKAT, providing access to chiral y-acetoxy ketones [84]. A variety of unsymmetrical 1,4-diols were transformed into the products in high to excellent yields (70-96%) and with ee values typically exceeding 85% (20). The chiral y-acetoxy ketones are useful as versatile building blocks for tetrahydrofurans and dihydrofurans. 

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