Chirale Synthesebausteine durch Kolbe‐Elektrolyse enantiomerenreiner β‐Hydroxy‐carbonsäurederivate. (R)‐ und (S)‐Methyl‐sowie (R)‐Trifluormethyl‐γ‐butyrolactone und ‐δ‐valerolactone

Abstract

Chiral Building Blocks for Syntheses by Kolbe Electrolysis of Enantiomerically Pure β‐Hydroxybutyric‐Acid Derivatives. (R)‐ and (S)‐Methyl‐, and (R)‐Trifluoromethyl‐γ‐butyrolactones, and ‐δ‐valerolactonesThe coupling of chiral, non‐racemic R* groups by Kolbe electrolysis of carboxylic acids R*COOH is used to prepare compounds with a 1.4‐ and 1.5‐distance of the functional groups. The suitably protected β‐hydroxycarboxylic acids (R)‐ or (S)‐3‐hydroxybutyric acid, (R)‐4,4,4‐trifluoro‐3‐hydroxybutyric acid (as acetates; see 1–6), and (S)‐malic acid (as (2S,5S)‐2‐(tert‐butyl)‐5‐oxo‐1,3‐dioxolan‐4‐acetic acid; see 7) are decarboxylatively dimerized or ‘codimerized’ with 2‐methylpropanoic acid, with 4‐(formylamino)butyric acid, and with monomethyl malonate and succinate. The products formed are derivatives of (R,R)‐1,1,1,6,6,6‐hexafluoro‐2,5‐hexanediol (see 8), of (R)‐5,5,5‐trifluoro‐4‐hydroxypentanoic acid (see 9,10), of (R)‐ and (S)‐5‐hydroxyhexanoic acid (see 11) and its trifluoro analogue (see 12, 13), of (S)‐2‐hydroxy‐ and (S,S)‐2,5‐dihydroxyadipic acid (see 23, 20), of (S)‐2‐hydroxy‐4‐methylpentanoic acid (‘OH‐leucine’, see 21), and of (S)‐2‐hydroxy‐6‐aminohexanoic acid (‘OH‐lysine’, see 22). Some of these products are further converted to CH3‐ or CF3‐substituted γ‐ and δ‐lactones of (R)‐ or (S)‐configuration (14, 16–19), or to an enantiomerically pure derivative of (R)‐1‐hydroxy‐2‐oxocyclopentane‐1‐carboxylic acid (see 24). Possible uses of these new chiral building blocks for the synthesis of natural products and their CF3 analogues (brefeldin, sulcatol, zearalenone) are discussed. The olfactory properties of (R)‐ and (S)‐δ‐caprolactone (18) are compared with those of (R)‐6,6,6‐trifluoro‐δ‐caprolactone (19).