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The major product obtained in the following reaction is

Dimethyl bis(hydroxymethyl)malonate treated with 2,2-dimethoxypropane and catalytic p-toluenesulfonic acid

  • A.Cyclic acetonide dione (Meldrum acid type ring) bearing two CH2OH groups on the ring carbon
  • B.Dimethyl 2,2-dimethyl-1,3-dioxane-5,5-dicarboxylate, a cyclic acetonide with two methyl ester groups
  • C.2,2-Dimethyl-1,3-dioxan-4-one ring with a methyl ester and a CH2OH group on the ring carbon next to the carbonyl
  • D.Spiro compound: 2,2-dimethyl-1,3-dioxan-4-one ring fused through a spiro carbon to a beta-lactone (oxetanone) ring

Answer: B

The major product is (B), the acetonide of the diol.

2,2-Dimethoxypropane with an acid catalyst is the standard reagent for protecting a 1,2- or 1,3-diol as an isopropylidene acetal. The two CH2OH groups of the starting diester form a 1,3-diol, and they close with the acetone unit into a six-membered 2,2-dimethyl-1,3-dioxane ring, releasing methanol. The two methyl ester groups are untouched, leaving dimethyl 2,2-dimethyl-1,3-dioxane-5,5-dicarboxylate.

(A) is a Meldrum's-acid-type ring formed from the carboxyl groups, which would need the free diacid. (C) and (D) are lactones formed from an ester and a hydroxyl group; acetal formation under these mild conditions does not convert the esters.

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