On the Resolution of Chiral Substrates by a retro‐Claisenase Enzyme: Biotransformations of Heteroannular Bicyclic β‐Diketones by 6‐Oxocamphor Hydrolase

Abstract

The enzyme 6-oxocamphor hydrolase (OCH) from Rhodococcus sp. NCIMB 9784 catalyses the cleavage of a carbon-carbon bond between two carbonyl groups in both mono-and bicyclic non-enolisable b-diketone substrates. In this mode OCH has been shown to effect the desymmetrisation of both bridged symmetrical bicyclic [2.2.1] and [2.2.2] systems and a series of 1-alkylbicycloA C H T U N G T R E N N U N G [3.3.0]octane-2,8-diones, yielding chiral substituted cyclopentanone and cyclohexanone products in high optical purity. In the present study, OCH has been challenged with a series of heteroannular substrates including 1-methylbicycloA C H T U N G T R E N N U N G [4.3.0]nonane-2,9-dione (7a-methylhexahydroindene-1,7-dione) in an effort to assess the competence of the enzyme for kinetic resolutions of asymmetric, racemic substrates. OCH was shown to catalyse the resolution of 1-methylbicyclo-A C H T U N G T R E N N U N G [4.3.0]nonane-2,9-dione with an E value of 2.9. The effect of increasing the length of the alkyl chain in the 1-position, or enlarging one of the rings, was to increase the enantioselectivity of the enzyme to 5.7 and 3.1 for the substrates 1-allylbicyclo-(9a-methyloctahydrobenzocycloheptene-1,9-dione) was not a substrate for OCH. These experiments constitute the first description of the resolution behaviour of such a retro-Claisenase enzyme, and suggest a maximum steric limit for substrate recognition by OCH.

Crotonases: Nature’s Exceedingly Convertible Catalysts

“…13,14 Fused 5,6 and 5,7 rings with non-enolisable 1,3-diketone systems also displayed reactivity with 6-OCH, producing enantioenriched products. 15 Limitations to the substrates accepted by 6-OCH remain, with all substrates thus far identified containing a cyclic 1,3-diketone scaffold. In addition, substrates require a nonenolizable centre between the two ketones, thereby excluding many simpler 1,3-diketones.…”

“…Pioneering work has hinted at this potential, including applications for the production of biofuels; the bioproduction of n -butanol has been engineered in bacteria via a metabolic pathway involving a crotonase . Crotonase enzymes are also candidates for engineering studies aiming to produce (intermediates of) bioactive molecules, including pharmaceuticals and other high-value chemicals, as well as chiral starting materials, as exemplified in substrate analogue studies with 6-oxocamphor hydrolase (6-OCH). − Of particular note from a commercial perspective has been the development of recombinant organisms which use bacterial enoyl-CoA hydratase/aldolase, a crotonase enzyme, coupled with a CoA-synthetase for conversion of ferulic acid to an expensive “natural” form of the flavor vanillin (Figure ). − More recently, a hydratase/lyase (VpVAN) catalyzing the conversion of ferulic acid itself into vanillin has been reported .…”

Stereoselective Synthesis of the Spirocyclic Ring System of the Sesquiterpene Spirolepechinene

“…Compound 6 would be [ To begin, allylation of 2-acetylcyclohexanone 2 was achieved via a tetrakis(triphenylphosphine)palladium (0) cata-lysed addition of allyl acetate [17,18] to give 2-acetyl-2-allylcyclohexan-1-one 7 in 87% yield (Scheme 1). Use of NaH and allyl bromide [19,20] also moderate yields, though not as consistently as the palladium catalysed allylation.…”

“…Compound 6 would be [a] Dr. by allylation and selective protection of commercially available 2.To begin, allylation of 2-acetylcyclohexanone 2 was achieved via a tetrakis(triphenylphosphine)palladium (0) cata-lysed addition of allyl acetate [17,18] to give 2-acetyl-2-allylcyclohexan-1-one 7 in 87% yield (Scheme 1). Use of NaH and allyl bromide [19,20] also moderate yields, though not as consistently as the palladium catalysed allylation.Next a selective ketal protection of the cyclic ketone was required to ensure correct aldol cyclisation in subsequent steps. This was achieved using ethylene glycol and a catalytic amount of p-toluene sulfonic acid in benzene, [17,21] giving the monoketal 6 (Scheme 1).…”

Biotransformations

“…The enzyme also showed some enantioselectivity with respect to kinetic resolution behaviour, catalysing the resolution of heteroannular bicyclic diketones, albeit with low E values. 65 Annu. Rep. Prog.…”