Chapter SRP, Problem 38P

Phenols generally are not changed on treatment with sodium borohydride followed by acidification to destroy the excess, unreacted hydride. For example, the 1,2-, 1,3-, and 1,4-benzenediols and 1,2,3-benzenetriol are unchanged under these conditions. However, 1,3,5-benzenetriol (phloroglucinol) gives a high yield of a product A that has these properties:

MS $\left(\text{m/z}\right)$: 110

IR $\left(c{m}^{-1}\right)$: 3250 (broad), 1613, 1485

${}^{1}H$ NMR ( $\text{δ}$ in DMSO): 6.15 (m, 3H), 6.89 (t, 1H), and 9.12 (s, 2H)

(a) What is the structure of A?

(b) Suggest a mechanism by which the above reaction occurred. (1,3,5-Benzenetriol is known to have more tendency to exist in a ketotautomeric form than do simpler phenols.)