Ring-contractive reaction

The reaction sequence is successful because reverse, ring-contraction reactions are unlikely and because only the final product contains a secondary lactam group, which is depro-tonated under the reaction conditions. 

A-Dinor-5a-cholestan-l-one has been prepared by Cava and Vogt by a multistep sequence involving, as the final ring contraction reaction, photolysis of 2-diazo-A-nor-5a-cholestan-1 -one. 

The enol ether 3-mcthoxy-4-phenyl-l-benzothiepin-5(4//)-one undergoes a similar ring contraction reaction at 80 °C. 

Nitrones derived from 2-azabicyclo[5.3.0]decane give quinolizidine compounds by photochemical Beckmann rearrangement which implies simultaneous ring expansion and ring contraction reactions. Intramolecular Schmidt reactions in 2(4-azidobutyl)-cyclopentanones also give quinolizidinone derivatives by ring expansion. Examples of both types of reactions are given in Sections 12.01.11.1 and 12.01.11.3, respectively. 

An unexpected ring contraction reaction has been reported. The attempted hydrolysis of 3-methoxycarbonyl-177-thieno[2,3-< ][l,3,4]thiadiazine 4,4-dioxide 77 under acidic conditions gave the ring-contracted thieno[2,3-<7][l,2,3]thia-diazole 78 instead of the expected carboxylic acid (Equation 24). A similar mechanism to the Hurd-Mori reaction has been proposed for this transformation <2000JHC191>. 

Triazolotriazepinones 475, obtained by reaction of 4,5-diamino-3-aryloxymethyl-l,2,4-triazoles 474, on heating with acetic anhydride undergo ring contraction reaction to yield l-acetyl-3-aryloxymethyl-6-methyl-pyrazolo[5,l-f][ 1,2,4]triazoles 476 (Scheme 54) <2003IJC(B)2054>. This type of transformation has been previously documented by other authors <1974JHC751>. 

Simoni et al. described <2000TL2699> that some fused tetrazoles readily participate in thermolytic ring contraction reactions which result in the formation of cyanopyrroles (Scheme 5). Thus, heating of tetrazolo[l,5- ]pyridine derivatives 14 at 150-170 °C yields the corresponding 2-cyanopyrrole 16. The process is believed to proceed via a nitrene intermediate 15. 

Ring contraction reaction of the benzthiazolo[2,3-f][l,2,4]triazin-one derivative 95 has been reported by Kuberkar et al. <2005JCM632> (Scheme 13). This compound when heated in aqueous sodium hydroxide underwent ring opening and a subsequent ring closure to result in formation of the fused [l,2,4]triazole ring system 96 in good yield (60%). 

A novel ring-contraction reaction which proceeds via an epi-sulfonium ion intermediate has been reported for the simple and regiospeciflc synthesis of mono-and symmetrically di-functionalized tetrathiamacrocycles, starting with the mono- or dichloro-substituted macrocycles that have one or two more ring atoms. 

A very powerful inhibitor, which is a bicyclic analogue of (33), is 6-acetamido-6-deoxycastanospermine (49), a compound synthesized from castanospermine and tested by Liu and co-workers [99]. This compound, amongst many other 6-modified castanospermine derivatives, was also prepared by the Furneaux team [100]. Furthermore, by controlled ring-contraction of suitably substituted castanospermine derivatives, these workers gained access to 8-acetamido-8-deoxyaustraline (50), a byciclic analogue of l-acetamido-l,2,5-trideoxy-2,5-imino-D-mannitol (44). This ring contraction reaction, which proceeds via a tricyclic aziridinium intermediate, is based on the same principle as the approach by Peter et al. to compound (45) [95]. 

Redmore, D. Gutsche, C. D. Carbocyclic Ring Contraction Reactions, in Advances in Alicyclic Chemistry Hart, H. Karabatsos, G. J., Eds. Academic Press New York. 1971 Vol. 3, p 1 -138. 

Other fluoroaromatic compounds such as fluorobenzene, trifluorotoluene and their derivatives may be elaborated to more complex aromatic compounds by directed metallation reactions (in itself, the subject of another large review [322-324]). Wakselman and co-workers described the conversion of 3-tri-fluoromethylphenol into 2-(trifluoromethyl)-l,3-cyclopentadienone, an intermediate they used to synthesise angularly trifluoromethylated steroid analogues [325]. The reaction, which involved an interesting ring contraction reaction, occurred with rather low efficiency (Eq. 122) [326]. 

CONVERSION OF MFA TO PARAHERQUAMIDES VIA A NOVEL PLATINUM-OXYGEN-MEDIATED RING CONTRACTING REACTION [33]... 

Our earlier conversion of MFA to PHA required 13 steps [7]. By employing a ring contracting reaction utilizing platinum and oxygen (Pt/02) at a key point in the synthesis, we were able to directly convert marcfortine A to the intermediate 16-oxoparaherquamide B (7), thereby eliminating six steps in our earlier synthesis. 

Fig. (38). A plausible mechanism for the Pt/02 meddiated ring contracting reaction...

In Section III,B it was pointed out that 2-phenylpyrimidines did not follow the general reduction path of pyrimidines but were reduced to 2-phenylpyrroles in a four-electron ring-contraction reaction.174 [Eq. (86)]. 

Dihydro compounds show reactions which parallel those of their aliphatic analogues provided that the aromatization or ring-contraction reactions just discussed do not interfere. 

Numerous instances exist of the preparation of benzofurans (poly-substituted by more or less complex groups) of ring-opening or ring-contraction reactions of natural products. 

Although molecular rearrangements of carbohydrate inflates are rare, they are not unknown [2,21,31]. Many of these rearrangements are ring-contraction reactions associated with treatment of Inflates with lithium triethylborohydride [see Eq. (10) 20,21]. Reported rearrangement reactions occurring when halide or azide ions are the nucleophiles are quite rare and are restricted to epimerization at centers next to a carbonyl group. Such reactions have been reported only for the more basic fluoride [32] and azide ions (Scheme 5) [33],... 

Ring Contraction Reactions Synthesis of a o-talitol Derivative. A mixture of disaccharide 47 (4.74 g, 6.4 mmol) and tetrabutylammonium benzoate (7 g, 19 mmol, 3 eq.) in 50 mL of toluene was heated 6 h at 80°C and then was evaporated to dryness. Flash... 

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