Enlargement of Ring

The oxidative rearrangement of 3-oxotabersonine (212) to 3-oxo-A14-vincamine (213) and the tetracyclic lactam (214) is analogous to the rearrangement of (189) to (190), (191), and (192).103 Another product (215), resulting from the enlargement of ring b by an unspecified reagent, is also reported to be obtained unfortunately, details of this work are at present inaccessible. 

Ring enlargement of A-acyl compounds, generally observed with oxaziridines, is observed only occasionally with diaziridines. Under more forced conditions of acylation with acetic anhydride, oxadiazolines like (139) were obtained (76MIP50800). A 4-nitrobenzoyl derivative rearranged at room temperature (76JOC3229). 

A formal ring enlargement of diazirines to five-membered rings is also observed with some hydrazones of ketodiazirines (65CB2509). On attempted preparation of hydrazones (201) from ketodiazirine (200) at 0 C the diazo compounds (202) are plausible intermediates since their transformation to aminotriazoles (203) is known. 

Ring enlargement of benzene derivatives by carbenes generated from diazo compounds (better in the presence of a Rh catalyst) Conversion of aldehydes to ketones by diazo compounds (Schlotterbeck) see also Ptau Planer... 

The procedure described is a modification of the directions of Prelog, Frenkiel, Kobelt, and Barman. Cyclodecanone has been prepared by the dehydration of sebacoin followed by catalytic hydrogenation, by the pyrolysis of the thorium or yttrium salt of nonane-1,9-dicarboxylie acid, and by the ring enlargement of cyclononanone, as well as by the reduction of sebacoin. ... 

It is the purpose of this chapter to describe a number of ring enlargement and simultaneous ring enlargement-ring contraction procedures which should be of general use. Recommended illustrative experimental procedures are given. 

To establish the structure of A-homo ketone (3b) Nelson and Schut repeated the TifFeneau ring enlargement of 5a-cholestanone [(Ib) (6a) (6b) -> (7)] and obtained a ketone identical with A-homo ketone (3b). 

In the latter process Dieckmann cyclization of diester (11) using high dilution conditions failed. However, A-homo-5a-cholestan-3-one (5b) identical to the product of diazomethane ring enlargement of (lb) was obtained in 35 % yield when diester (11) was hydrolyzed to the bis-homo diacid and this converted to the thorium salt and pyrolyzed. 

Jones and Price"" therefore concluded that diazomethane ring enlargement of 17j5-hydroxy-5a-androstan-3-one (Ic) proceeds with predominately C3—C4. migration, in contrast to the 5a-cholestane series which differs from (Ic) only in the 17-substituent. 

Ring enlargement of 17jS-hydroxy-5jff-androstan-3-one (12) with diazomethane gives 17j8-hydroxy-A-homo-5j8-androstan-3-one (13) and 17)8-hydroxy-A-homo-5j8-androstan-4-one (14) in a total yield of 72%. 

Tiffeneau ring enlargement of 5a-deuterio ketone (59) gives the required 3yS-hydroxy-B-homo-5a-deuteriocholestan-7-one acetate (60). The 6a-bromo-3 -hydroxy-B-homo-5a-deuteriocholestan-7-one acetate (61) obtained by bromination is then treated with lithium bromide in dimethylformamide to give the deuterium free a,j5-unsaturated ketone (62). The loss of deuterium bromide proves that bromination takes place at C-6 and that the carbonyl group must therefore be located at C-7. 

Clemmensen reductions of a-aminoketones that proceed with ring enlargement or ring contraction are presumed to proceed by an iminium intermediate. This reaction has been examined in detail (137-139), and an example is given in the conversion of (94) to an iminium intermediate (95), which is reduced to 96. 

Recently, the ring enlargement of 4-hydroxy-2-cyclobutenones 5 was promoted by PhI(OAc)2, a popular and accessible hypervalent iodine reagent (99JOC8995). Thus, when 5a-c (R = Me, Bu, Ph) were treated with a slight excess of PhlfOAcja in dichloromethane at room temperature, the 5-acetoxy-3,4-diethoxyfuranones 13... 

Ring enlargement of a-nitrQcyclo ilkanQnes has been extensively used fot the synthesis of mactocyclic compounds, as exemplified in Eq 5 22 ... 

Thermolysis of 3-oxaquadricyclanes in which the bond between Cl and C2 is part of a cyclohexane structure gives a,/J-annulated oxepins 11 in good yield.133 The enlargement of the an-nulated ring favors the arene oxide structure.134... 

By Acid-Catalyzed Ring Enlargement of Benzo A cyck>propa e]thiopyrans... 

The photochemical ring enlargement of quinoline 1-oxides provides 3,1-benzoxazepines 1 (see Houben-Weyl, Vol.4/5b, p 12971T).24... 

For the synthesis of ring-enlarged epothilone analogs by the same principle, see (a) Rivkin A, Njardarson JT, Biswas K, Chou T-C, Danishefsky SJ (2002) J Org Chem 67 7737 (b) Rivkin A, Biswas K, Chou T-C, Danishefsky SJ (2002) Org Lett 4 4081... 

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