Diastereoselection in reactions of pinanediol dichloromethaneboronate

Article abstract of DOI:10.1021/om50005a010

Dichloromethaneboronic acid was converted to its (+)-pinanediol ester (1) (92% ee), which was treated with lithium or Grignard reagents to form the corresponding intermediate borate complexes (2), diastereomeric with the borates 3 obtained by adding (dichloromethyl)lithium to the corresponding (+)-pinanediol boronic esters. Rearrangement of the lithium n-butylborate complex 2a to the 1-chloropentane-1-boronic esters (4a and 5a) followed by treatment with phenyllithium to form the 1-phenylpentane-1-boronic esters (6a and 1a) and finally oxidation with alkaline hydrogen peroxide yielded predominantly (R)-(+)-1-phenyl-1-pentanol (8r) (0.92 × 31% ee). This low enantiomeric excess contrasts with the 80% ee obtained previously in a similar sequence from the oppositely assembled borate complex 3a. Starting with the phenylborate complex 2b, rearranging, treating with butyllithium, and oxidizing yielded (S)-(-)-1-phenyl-1-pentanol (8s) (0.92 × 37% ee). The magnesium isobutylborate complex 2c was rearranged and converted to the 1-acetamido-3-methylbutane-1-boronic esters (9c and 10c), which showed separate NH peaks in the ¹H NMR, ratio 34/66, opposite the stereoselectivity for 2a and 2b. Similarly, the methyl complex 2d yielded a 22/78 ratio of 9d and 10d. Zinc chloride catalysis of the rearrangements of 2c and 2d yielded 9/10 ratios of 92/8 and 51/49, respectively.