An enantioselective synthesis of and 1.2-diok via the reactions of aldehydes and the tartrate ester modified y(akoxysiiyl)allylboronate 2 has been developed. This method is most Mective in double asymmemk reuctions with chiral akiehydes.
The stereoselective synthesis of syn and anti 1,2diol units is an important operation in the synthesis of polyhydmxylated natural pr0ducts.t Methods that enable these units to be generated in concert with a C-C bond forming event are particularly attractive, especially if the method is highly stemose1ective.l~ While considerable success has been realized in the synthesis of syn 1.2~diol monoethers via the addition of various 'I_ alkoxyallylmetal reagents (Met = B, Al, Sn) to aldehydes, 3*4Jd the stereocontrolled synthesis of the anti diastemomets has proven to be a much more difficult problem. 3&e*56 Notable exceptions are Tamao's y-(diallqlaminosilyl)allyl zinc reagent and Talcai's @koxyallylchromium species that provide convenient, stereoselective access to anti 1,2-diol derivatives.5ef (E)-)LAlkoxyallylboron reagents, however, have not been widely applied in this stereochemical series owing to the difficulty of synthesis and the configurational instability of the Q-alkoxyallyl anion pmcursors.3c~
We report herein an enantioselective synthesis of anti 12-diols via the reactions of aldehydes and the chiral allylboronate 2. This method takes advantage of (i) the well-known preference of l-(ttialklysilyl)allylanions to undergo electrophilic substitution at the y-position to give products containing (PI)-vinylsilane units (c.f., 1 + 2),7 (ii) the tendency of (E)-~substituted allylboronates, including (E)-r-(trialkylsilyl)allylboronates.* to react with aldehydes via cyclic transition states to provide the 3,4-anti diastereomers with high stereoselectivity (cf., 2 + 3),38*9 (iii) the enantioselectivity of tartrate ester auxiliaries in allylboration reactions.89 and (iv) the stereoretentive oxidation of C-SiRfi units to alcohols.~Jo While the present methodology is well precedented by the earlier study of Tamao and Ito,Se it constitutes a significant improvement in two respects. First, it is enantioselective by virtue of the tam-ate auxiliary, thus enabling much greater stereochemical control to be exercised in reactions with chiral aldehydes (i.e., double asymmetric synthesis).11 Second, the "metal alkoxide" produced in the reactions of 2 and aldehydes is a nonbasic borate ester that has no tendency to undergo Petersen eliminations prior to workup. WV-2 2) (iPrOhB, -78OC 1 WHO. toluene RX' HA. W, KHCOS c -780~. 4Asieves 3 hH MeoH. THF, 23°C bH 4