Steroidal enamines

The halogenation of enamines is formally analogous to protonation with salt formation. Thus the steroidal enamine (164) undergoes bromination (7/5) to give the j8-bromo iminium bromide (165), which is readily hydrolyzed to the /5-bromo aldehyde (166). 

Other interesting synthetic applications of the ketone-derived enamine alkylation are found in the monomethylation of steroid enamines (249), extension of the benzylation reaction (250) to a ferrocene derivative (251), the use of a-bromoesters (252) and ketones (252) or their vinylogues (25J), in the syntheses of alantolactone (254-256), isoalantolactone (257), and with a bridged bis-enamine (258). The use of bifunctional alkylating agents is also seen in the introduction of an acetylenic substituent in the synthesis of the characteristic fragrant constituent of jasmine (259), the synthesis of macrocyclic ketolactones (260), the use of butyrolactone (261), and the intermolecular or intramolecular double alkylations of enamines with dihalides (262). 

S. Nakanishi, R.L. Morgan, E.V. Jensen, 4-Fluorinated steroids from the reaction of perchloryl fluoride with steroid enamines, Chem. Ind. (London) (1960) 1136-1137. 

Ionized primary enamines of low energy may also be reaction intermediates in the loss of ammonia. Scheme 34 shows a mechanism proposed for the extensive loss of ammonia from low-energy ionized 3-amino steroids. Enamine 63 may lose ammonia after two successive 1,4 H-transfers. The participation of the H at C(5) has been established by D-labelling42. Only higher energy ions lead to m/z 82 fragment ions through the ionized dienamine 64,... 

In fact steroidal enamines react with a mixture of ozone and oxygen to give the corresponding expected ketones, progesterone in the example of Scheme 2439. 

A particularly complicated reaction is reported to occur43,47 with a steroidal enamine. Formation of the products can be explained in terms of the intermediacy of an oxaziridinium salt. 

Enamines are oxidized by peracids to yield, after hydrolysis, the corresponding a-hydroxy ketones, as shown in Scheme 28 for a steroidal enamine. ... 

Allylic amines are coupled to halides giving either allylic amines or enamines depending on the reaction condition. Reaction of steroidal dienyl triflate with Boc-diprotected allylamine affords allylamine. Use of AcOK as a base is crucial for the clean coupling[102]. The tert-allylic amine 123 reacts with an aryl halide to give the enamine 125 in DMF and allylic amine 124 in nonpolar solvents[103]. 

Extensions of this concept have utilized enamine hydrolysis (171, X = R N) and the quenching of the enolate anion (171, X = O ) e.g. ref. 353). a,(i-Un-saturated ketones are usually more stable than their p,y counterparts, but there are notable exceptions to this, and in such cases the deconjugated ketone may be isolated from the equilibrated system. For example, retro steroids (9, 10a) have a large proportion of A -3-ketone at equilibrium, and 17-ketones yield the more stable A -system on treatment with acid. ... 

Steroidal A -dienamines are formed with a high degree of selectivity in the presence of 17- or 20-ketones. Preparation of morpholine and pyrrolidine enamines is achieved by azeotropic removal of water with or without a catalyst. Pyrrolidine enamines are also formed efficiently by addition of the base to the hot solution of the ketone in methanol followed by immediate cooling. ... 

The enamines, enol ethers and enol acetates of A -3-keto steroids provide important substrates for fluorination with FCIO3. Reaction of such A -enol ethers and acetates (6) with perchloryl fluoride results in 6a- and 6jff-fluoro-A -3-ketones (7) and (8), the latter representing the more abundant isomer. Tetrahydrofuran or dioxane-water mixtures appear to be particu-... 

A unique A -derivative, capable of conversion to 17a-hydroxy-20-keto steroids, is one containing nitrogen attached to C-20 e.g., enamines or N-acetylenamines). ... 

Enamines of 20-keto steroids cannot be prepared directly by standard methods. 

Cyanogen azide is a useful reagent for conversion of pyrrolidine enamines of 3-keto steroids to A-norsteroids. " Ring contractions can be carried out in the presence of 17j5-hydroxy, 17j -acetoxy, 20-keto groups and isolated double bonds. In a typical procedure, 17j -hydroxy-5a-androstan-3-one (partial formula 8) is converted into the enamine (9) by pyrrolidine in benzene... 

Reactions of fluoroxytrifluoromethane with enol ethers, enol acetates, and enamines [/, 2, 3] are very useful, especially for the preparation of steroidal ct-fluoTo ketones (Table 2, entries 1, 3, 5, 6, and 7) [7] (equation 12)... 

In the steroidal series Heyl and Herr (40) have reported that the pyrrolidine enamine of 5a-cholestan-3-one consisted of a mixture of and A isomers (78),... 

That an aminal is a necessary intermediate was first questioned by Herr and Heyl (2). They found that by using a slight excess of amine the yield of the enamine from two of the steroidal aldehydes studied was 84%. Also, the -fluorocnamines discussed earlier are formed in 60-90% yield from equimolar amounts of the 8-fluoroaldehyde and secondary cyclic amine (27). However, neither of these studies was specifically designed to show whether or not aminals were intermediates. 

N-acyl enaminc (104, R = CHjCHj) gave an unstable enamine (106) which decomposed readily to 3-cholestanone. The steroidal N-acetyl enamines (107 and 108, R = C HjCHj) can be reduced by lithium aluminum hydride in tctrahydrofuran to the corresponding enamines (109, R = CJH5CH2) in 90 and 68% yield, respectively 100). Attempts to reduce the enamide (107, R = CH3) led to the formation of the impure enamine (109, R = CHj), which decomposed to the hydroxy ketone (110). 

Enamines from steroidal ketones have been fluorinated by means of perchloryl fluoride (119,120) to give the a-fluorinated ketones. Theenamino ketone (49) was brominated to give a salt from which the bromoenamino ketone (175) was isolated on treatment with dilute carbonate solution (121). 

In the alkylation of enolate anions, a mixture of mono- and polyalky lation produets is usually obtained, and when enolization of a di-a-methylene ketone is possible toward both sides, a mixture of di-a- and a,a -dialkylation products ean be expeeted. Thus the enamine alkylation sequenee beeomes partieularly attractive when eontrolled monoalkylation is imperative beeause of difficulties in separation of a mixture of alkylation produets. One of its first synthetie applications was in the reaetions of /8-tetralones with alkyl halides. Yields in exeess of 80% were usually found 238-243) in these reaetions, which make valuable intermediates for steroid and diterpene syntheses more aecessible. 

The acylation of enamines has been applied to the use of long-chain acid chlorides (388) and particularly to the elongation of fatty acids (389-391) and substituted aliphatic acids (392). The method has been used in the synthesis of the antineoplastic cycloheximide and related compounds (393-395) and in the acylation of steroids (396). Using an optically active chlorocarbonate, an asymmetric synthesis of lupinine could be achieved (397). 

The double a substitution of enamines with propane-1,3-dithiol ditosylate has provided a route for the introduction of a thioketal, which has found use as a blocking group in modified steroid 476) and terpene 477) syntheses. 

The most extensive use of enamine halogenations has, hctwever, been in the attachment of fluorine to the steroid skeleton (499-503). The formation of a ]6-fluoro-17-ketosteroid by the reaction of perchlorofluoride with a 17-enamide has also been reported (504). 

Greater stereoselectivity for the formation of equatorial amines has been found in the reduction of enamines with formic acid or formamides (553-559). The selective formation of 3-a-amino-5- -steroids by this method and of 3- 3-amino-5- 3-steroids by catalytic reduction (5<50) of the corresponding enamines is of interest. 

The use of enamines as protective groups seems largely to be confined to steroid chemistry, where they serve (in their protonated form) to protect the A-B enone system from bromination and reduction. A large body of literature exists on the preparation and chemistry of enamines, which are easily hydrolyzed with water or aqueous acid. 

The first step of the Robinson annulation is simply a Michael reaction. An enamine or an enolate ion from a jS-keto ester or /3-diketone effects a conjugate addition to an a-,/3-unsaturated ketone, yielding a 1,5-diketone. But as we saw in Section 23.6,1,5-diketones undergo intramolecular aldol condensation to yield cyclohexenones when treated with base. Thus, the final product contains a six-membered ring, and an annulation has been accomplished. An example occurs during the commercial synthesis of the steroid hormone estrone (figure 23.9). 

Intermolecular [4C+2S] cycloaddition reactions where the diene moiety is contained in the carbene complex are less frequent than the [4S+2C] cycloadditions summarised in the previous section. However, 2-butadienylcarbene complexes, generated by a [2+2]/cyclobutene ring opening sequence, undergo Diels-Alder reactions with typical dienophiles [34,35] (Scheme 59). Also, Wulff et al. have described the application of pyranylidene complexes, obtained by a [3+3] cycloaddition reaction (see Sect. 2.8.1), in the inverse-electron-demand Diels-Alder reaction with enol ethers and enamines [87a]. Later, this strategy was applied to the synthesis of steroid-like ring skeletons [87b] (Scheme 59). 

The product in entry 1 of Scheme 2.10 is commonly known as the Wieland-Miescher ketone and is a useful starting material for the preparation of steroids and terpenes. The Robinson annulation to prepare this ketone can be carried out enantioselectively by using the amino acid L-proline to form an enamine intermediate. The 5-enantiomer of the product is obtained in high enantiomeric excess.89 This compound and the corresponding product obtained from cyclopentane-1,3-dione90 are key intermediates in the enantiose-lective synthesis of steroids.91... 

Whereas Knoevenagel condensation of 17-oxo-steroids occurs readily, 16a-substituted-16-methylene-17-oxo-steroids are unreactive. The enamine (111), however, reacted with CH(CN)2 to give mainly the aromatic compounds (112) by the mechanism outlined in Scheme 2. ... 

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