Enantiopure (S)‐(1‐2H)ethylbenzene was prepared in two steps from optically active (S)‐1‐phenylethanol via (R)‐(1‐chloroethyl)benzene (two inversions of configuration). Since the value for the specific rotation [α] is very low for the enantiomers of (1‐2H)ethylbenzene, the enantiopurity of the synthetic product could not be determined with certainty by polarimetry. Therefore, bis‐sulfonamides were prepared by twofold chlorosulfonation (para and ortho) of (S)‐(1‐2H)ethylbenzene and subsequent amidation with (R)‐ and (S)‐α‐phenethylamine. For both diastereoisomers, the (R,R,S)‐ and the (S,S,S)‐sulfonamides, 92 % de was determined by 1H NMR spectroscopy. Therefore, it could be concluded, that (S)‐(1‐2H)ethylbenzene had been obtained with 92 % ee.

The constitutions and absolute configurations of two previously unknown intermediates, (1S,2S,4S)‐2‐hydroxy‐4‐isopropylcyclohexane‐1‐carboxylate and (S)‐3‐isopropylpimelate, of anaerobic degradation of p‐cymene in the bacterium Aromatoleum aromaticum pCyN1 are reported. These intermediates (as CoA esters) are involved in the further degradation of 4‐isopropylbenzoyl‐CoA formed by methyl group hydroxylation and subsequent oxidation of p‐cymene. Proteogenomics indicated 4‐isopropylbenzoyl‐CoA degradation involves (i) a novel member of class I benzoyl‐CoA reductase (BCR) as known from Thauera aromatica K172 and (ii) a modified β‐oxidation pathway yielding 3‐isopropylpimeloyl‐CoA analogously to benzoyl‐CoA degradation in Rhodopseudomonas palustris. Reference standards of all four diastereoisomers of 2‐hydroxy‐4‐isopropylcyclohexane‐1‐carboxylate as well as both enantiomers of 3‐isopropylpimelate were obtained by stereoselective syntheses via methyl 4‐isopropyl‐2‐oxocyclohexane‐1‐carboxylate. The stereogenic center carrying the isopropyl group was established using a rhodium‐catalyzed asymmetric conjugate addition. X‐ray crystallography revealed that the thermodynamically most stable stereoisomer of 2‐hydroxy‐4‐isopropylcyclohexane‐1‐carboxylate is formed during p‐cymene degradation. Our findings imply that the reductive dearomatization of 4‐isopropylbenzoyl‐CoA by the BCR of A. aromaticum pCyN1 stereospecifically forms (S)‐4‐isopropyl‐1,5‐cyclohexadiene‐1‐carbonyl‐CoA.

The constitutions of seven metabolites formed during anaerobic degradation of n‐hexane by the denitrifying betaproteobacterium strain HxN1 were elucidated by comparison of their GC and MS data with those of synthetic reference standards. The synthesis of 4‐methyloctanoic acid derivatives was accomplished by the conversion of 2‐methylhexanoyl chloride with Meldrum's acid. The β‐oxoester was reduced with NaBH4, the hydroxy group was eliminated, and the double bond was displaced to yield the methyl esters of 4‐methyl‐3‐oxooctanoate, 3‐hydroxy‐4‐methyloctanoate, (E)‐4‐methyl‐2‐octenoate, and (E)‐ and (Z)‐4‐methyl‐3‐octenoate. The methyl esters of 2‐methyl‐3‐oxohexanoate and 3‐hydroxy‐2‐methylhexanoate were similarly prepared from butanoyl chloride and Meldrum's acid. However, methyl (E)‐2‐methyl‐2‐hexenoate was prepared by Horner–Wadsworth–Emmons reaction, followed by isomerization to methyl (E)‐2‐methyl‐3‐hexenoate. This investigation, with the exception of 4‐methyl‐3‐oxooctanoate, which was not detectable in the cultures, completes the unambiguous identification of all intermediates of the anaerobic biodegradation of n‐hexane to 2‐methyl‐3‐oxohexanoyl coenzyme A (CoA), which is then thiolytically cleaved to butanoyl‐CoA and propionyl‐CoA; these two metabolites are further transformed according to established pathways.

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