Ring contraction compounds

Fig. 14 Hydrogenolysis on metal catalysts product from ring opening reactions of Cl ring contraction compounds and their corresponding research octane number and motor octane number. Adapted from ref. 100.

Depending on the reaction medium some cyclic alkaloids behave as ring enlarged or as ring contracted compounds. In Scheme VIII/19 indole alkaloids vomicine and perivine, the isoquinoline alkaloid protopine, and pyrrolizidines are given with their ring contracted forms. 

Irradiation (254 nm) of the imide (430) brings about partial isomerization of the starting material to the trans-isomer (431). This product is accompanied by the ring-contracted compounds (432) and (433). Secondary irradiation of the... 

T. Tsuchiya, K. Ajito, S. Umezawa, and A. Ikeda, Reactions on methyl 2-deoxy-2-trifluoro-acetamido-3-O-trifluoromethylsulfonyl-a-D-glucopyranoside derivatives Formation of ring-contraction compounds, Carbohydr. Res., 126 (1984) 45-60. 

Although numerous examples of successful RCM reactions have been demonstrated, a few limitations and/or side-reactions have been uncovered. Some cases where the RCM reaction proceeds with complications are depicted in Scheme 18. Sometimes RCM reactions are competitive with alkene isomerization. For example, the unexpected formation of 162 from precursor 160 was attributed to alkene isomerization (affording 161), followed by RCM to afford the ring-contracted compound 162. Later investigators went on to exploit this observation for the synthesis of cyclic enol ethers. Treatment of the allyl ether 164 with a ruthenium carbene complex catalyst affords the RCM... 

Deamination of primary amines to rearranged alcohols (via diazonhjm compounds) with ring contraction or enlargement for alicyclic amines... 

The tetracyclic y-carboline derivative 233 was obtained via an interesting ring contraction when the seven-membered ring compound 232 was heated with selenium. 

A detailed study of the dehydrogenation of 10.1 l-dihydro-5//-benz[6,/]azcpinc (47) over metal oxides at 550 C revealed that cobalt(II) oxide, iron(III) oxide and manganese(III) oxide are effective catalysts (yields 30-40%), but formation of 5//-dibenz[7),/]azepinc (48) is accompanied by ring contraction of the dihydro compound to 9-methylacridine and acridine in 3-20 % yield.111 In contrast, tin(IV) oxide, zinc(II) oxide. chromium(III) oxide, cerium(IV) oxide and magnesium oxide arc less-effective catalysts (7-14% yield) but provide pure 5H-dibenz[b,/]azepine. On the basis of these results, optimum conditions (83 88% selectivity 94-98 % yield) for the formation of the dibenzazepine are proposed which employ a K2CO,/ Mn203/Sn02/Mg0 catalyst (1 7 3 10) at 550 C. 

Nitroso-5//-dibenz[/j,/ azepine (see Section 3.2.1.5.4.1.) in methanolic hydrochloric acid undergoes rearrangement and ring contraction to a mixture of acridine (59%), acridine-9-carbaldehyde (trace), and 2-nitro-5//-dibenz[/ ,/ azepine (3% mp 176-178 C).184 However, in acetone and hydrochloric acid, the aldehyde (57 %) becomes the major product. On thermolysis, (or photolysis in the presence of oxygen), in hydroxylic solvents, the nitroso compound yields mainly acridine (36-76%) together with minor amounts of either 2-nitrodibenzazepine (4-6% by thermolysis) or acridine-9-carbaldehyde (18% by photolysis). However, in non-hydroxylic solvents, e.g. cumene, acridine-9-carbaldehyde (64%) is the major product. 

An analogous ring contraction occurs in the reaction of the methylsulfanyl compound 3 with morpholine to give the benzoxazole derivative 4.37... 

Compound 15 is prepared in two steps acid-catalyzed condensation of the pyrimidinediamine with ethyl aceloacelale provides ester 14, which cyclizes under strongly basic conditions. The pyrimidodiazepinone 15 undergoes ring contraction to 7-isopropenyl-7//-purin-8(9//)-one (16) on prolonged heating under basic conditions.297... 

Secondly, base-induced rearrangements of porphycenes2-5,6 have been observed to yield isocorroles by ring contraction. Octaethylporphycene 6 when treated with potassium hydroxide in dimethyl sulfoxide/bis(2-methoxyethyl) ether (diglyme) was found to give a mixture of octa-ethylisocorrolecarbaldehydc 7 and octaethylisocorrole 8. The ratio of the compounds varies depending on the reaction conditions. 

Reaction of the cyclic sulfoxide 1176 with trimethylchlorosilane (TCS) 14 in CH2CI2 affords the unsaturated sulfide 1177, in 86% yield, and HCl and HMDSO 7 whereas the ester 1178 gives rise to 72% 1179, 16% isomer 1180, and, via 1181, the chloro compounds 1182 and the ring contracted 1183 in 7% yield [17, 18] (Scheme 8.5). 

Tetrahydroanthracene is the linear benzologue of Tetralin. This compound behaves identical to Tetralin in the presence of dibenzyl with respect to ring contraction, giving a single methyl signal at 19.6 ppm (Figure 5). Contrary to... 

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