G. Denis Meakins

Microbiological hydroxylation of steroids. Part VIII. The pattern of monohydroxylation of diketones and keto-alcohols derived from 5α-androstane with cultures of the fungus, Rhizopus nigricans

Diketones and keto-alcohols derived from 5α-androstane are readily converted into monohydroxy-derivatives by Rhizopus nigricans. Varying the positions and oxidation levels of the oxygen functions leads to hydroxylation at different positions. Substrates with one oxygen substituent in each of the terminal rings are attacked at position 11 or 7; those with one group in rings B or C are hydroxylated at position 16 if the second group is in ring A, and at position 3 if the second group is in ring D.The results are rationalised by assuming the presence of three dual-purpose sites on the enzyme surface capable of binding to the steroidal oxygen groups and of hydroxylating those positions of the steroid nucleus which come into their vicinity.

Microbiological hydroxylation of steroids. Part VII. The pattern of dihydroxylation of mono-oxo-5α-androstanes and -5α-estranes with the fungus Rhizopus nigricans

Although steroidal monoketones with the carbonyl group in ring B or C are relatively unreactive towards Rhizopus nigricans, 2-, 3-, 16-, and 17-ketones give modest yields of dihydroxy-derivatives. The position of the carbonyl group influences the direction of the hydroxylation process: comparison of the 11,16-dihydroxylation of 3-ketones with the 3,7-dihydroxylation of 17-ketones suggests that a reversal effect is operating.16-Hydroxylation, not previously recorded with this fungus, occurs commonly with the present androstane and estrane derivatives, i.e. steroids lacking side-chains at position 17. Estr-4-en-3-one gives three 16-oxygenated products (total yield 68%), the main one being the 10β,16β-dihydroxy-Δ4-3-ketone.

N′-substituted N-acyl- and N-imidoyl-thioureas: preparation and conversion of N′,N′-disubstituted compounds into 2-(N,N-disubstituted amino)thiazol-5-yl ketones

Known methods were developed to give convenient general procedures for preparing N-acyl-N′-mono and -N′,N′-disubstituted thioureas from acid chlorides, and related N-imidoyl thioureas from imidoyl chlorides. In the products from three acid chlorides and ammonium thiocyanate the acyl isothiocyanates did not appear to be accompanied by the isomeric thiocyanates.Treatment of N-(anilino)benzylidene-N′,N′-disubstituted thioureas with chloroacetone in the presence of triethylamine leads to 5-acetyl-4-phenyl -2-(N,N-disubstituted amino)thiazoles. In contrast, the corresponding N-benzoyl thioureas form only small amounts of these compounds; the main products are the 5-benzoyl-4-methyl isomers, and this unexpected outcome requires a revision of the literature. It is thought that formation of the 5-benzoyl-4-methylthiazoles involves N–C(4) fission of a cyclic intermediate to give an open-chain intermediate in which nucleophilic attack can occur at either the acetyl or the benzoyl group. One of the latter intermediates was generated directly from 2-acetyl-2-bromoacetophenone and N-methyl-N-phenylthiourea, and found to give the 5-benzoyl-4-methyl - and 5-acetyl-4-phenyl-thiazoles as the major and minor products, respectively.

The preparation of thiazole-4- and -5-carboxylates, and an infrared study of their rotational isomers

Convenient general procedures have been developed for preparing series of thiazole-4- and -5-carboxylates containing alkyl and halogeno substituents. While both series of esters show i.r. carbonyl doublets caused by rotational isomerism, the more intense absorptions of the 4-carboxylates are the lower wavenumber components, whereas those of the 5-carboxylates are the higher wavenumber components. In both series the stronger bands arise from the thermochemically more stable forms; identification of these forms as the carbonyl O,S-syn-s-trans rotamers is more certain with the 4-carboxylates than with the 5-carboxylates.

Microbiological hydroxylation. Part 23. Hydroxylations of fluoro-5α-androstanones by the fungi Calonectria decora, Rhizopus nigricans, and Aspergillus ochraceus

A series of monofluoro- and gem-difluoro-5α-androstanones and the parent ketones were incubated, under standard conditions, with the fungi named in the title. The results may be rationalised by comparing the positions of the fluorine atoms in the substrates with those of the favoured hydroxylation sites in the parent ketones. With few exceptions hydroxylation does not occur at, or adjacent to, the carbon to which a fluorine substituent is attached even though one of these centres is a favoured site (in the parent ketone). In such cases hydroxylation is usually diverted to an alternative position. Where the favoured site is more distant from the fluorine substituent(s) the behaviour of a fluoro-ketone resembles that of its parent. Hydroxylation of several fluoro-ketones by Aspergillus ochraceus gives the 11 α-hydroxy-derivatives cleanly and in yields which are satisfactory for preparative work.

Substituted imidazo[2,1-b]thiazoles from 2-aminothiazoles and α-bromo ketones: efficient preparation and proof of structure

The salts formed from α-aminothiazoles and α-bromo ketones (RCOCH2Br) have been basified, and the products converted into amides. Examination of the amides established that they are 2-acylimino-2,3-dihydrothiazoles rather than 2-acylaminothiazoles. Thus the α-bromo ketones attack the endo-N of the 2-aminothiazoles, and the imidazo[2,1-b]thiazoles obtained by cyclising the salts have the substituent (R) of the bromo ketone at position 6. Efficient procedures have been developed for preparing a range of imidazo[2,1-b]thiazoles.Related reactions of 2-aminothiazoles, with an α-bromo aldehyde and with ethyl bromoacetate, have been studied.

Microbiological hydroxylation. Part XIV. Hydroxylation in the terminal rings of dioxygenated 5α-androstanes with the fungi Wojnowicia graminis and Ophiobolus herpotrichus

Dioxo-5α-androstanes having one keto-group in a terminal ring (at position 3 or 2, or at position 17) and the second at a middle ring position (7 or 11) are hydroxylated in the other terminal ring (at 17 or 16, or at 3 or 2) by the fungi Wojnowicia graminis and Ophiobolus herpotrichus. Efficient transformations include the 2α-hydroxylation of 5α-androstane-7,17-dione (with W. graminis), the 16-substitution of the 3,11-dione (with O. herpotrichus), and the 17β-hydroxylation of 3,7-dioxygenated substrates (with both fungi).

Microbiological hydroxylation of steroids. Part IX. Hydroxylation of diketones and keto-alcohols derived from 5α-androstane with the fungi Rhizopus arrhizus and Rhizopus circinnans. Steroidal 18- and 19-proton magnetic resonance signals

The hydroxylations of seven diketones and keto-alcohols derived from 5α-androstane with Rhizopus arrhizus and Rhizopus circinnans are similar to, but not identical with, those observed previously using Rhizopus nigricans. R. circinnans is useful for introducing a 4α-hydroxy-group into 5α-androstane-11,17-dione.N.m.r. shift values are given for the influence on steroidal 18- and 19-H signals of various single substituents, and of systems containing two or more substituents whose effects are not additive.

The Hantzsch thiazole synthesis under acidic conditions: change of regioselectivity

The condensation of α-halogeno ketones with N-monosubstituted thioureas in neutral solvent leads exclusively to 2-(N-substituted amino)thiazoles. In the present work it was shown that under acidic conditions mixtures of 2-(N-substituted amino)thiazoles and 3-substituted 2-imino-2,3-dihydrothiazoles are formed. (The isomers were distinguished by characteristic differences between their 5-H 1H n.m.r. signals and the i.r. CO bands of their trifluoroacetate derivatives.) The proportions of the 2-imino-2,3-dihydrothiazoles in the mixtures are influenced by experimental features and by the structures of the starting materials. Reactions in 10M-HCl-EtOH (1 :2) at 80 °C for 20 min were found to be the most efficient for generating 2-imino-2,3-dihydrothiazoles; in the most favourable case 2-imino-3,4-dimethyl-2,3-dihydrothiazole was obtained in 73% yield.A possible explanation of the results is discussed.

Esters of furan-, thiophen-, and N-methylpyrrole-2-carboxylic acids. Bromination of methyl furan-2-carboxylate, furan-2-carbaldehyde, and thiophen-2-carbaldehyde in the presence of aluminium chloride

Convenient routes from readily available starting materials to furan- and thiophen-2-carboxylic acids with substituents (mainly halogen atoms) at various positions are described. A series of esters (methyl, ethyl, t-butyl, and 17-oxo-5α-androstan-3β-yl) have been prepared from these acids and from the unsubstituted heterocyclic 2-carb-oxylic acids.Methyl furan-2-carboxylate reacts with bromine in the presence of aluminium chloride to give the 4,5-dibromo-ester. The presence of 1,2-dichloroethane changes the course of the reaction, the product then being the 4-bromo-5-chloro-ester. Furan-2-carbaldehyde shows the opposite behaviour, giving as the main products the 4-bromo-5-chloro-aldehyde in the absence of a solvent and the 4,5-dibromoaldehyde when 1,2-dichloroethane is present. Careful control of conditions is necessary in the preparation of 4-bromothiophen-2-carbaldehyde from thiophen-2-carbaldehyde, bromine, and aluminium chloride.