Ring contraction selective

Another useful route to cyciopentanes is the ring contraction of 2-bromo-cydohexanones by a Favorskii rearrangement to give csrdopcntanecarboxylic acids. If a 0 dibromoketones are used, ring opening of the intermediate cydopropanone leads selectively to, y-unsaturated carboxylic acids (S.A, Achmad, 1963, 1965 J. Wolinsky, 1965). 

Ring-contraction methods described in this chapter have been selected from the many examples reported in the literature on the basis of outstanding reliability and utility. When several adequate methods are available for accomplishing the same transformation only one, and hopefully the best, experimental procedure is presented. The author admits a predilection for methods which in his own experience have proved useful. 

Dimethylfuran, in a sensitized reaction, gave 1,3-dimethylcyclopropene (the main product), isoprene, cis- and rran.s-l,3-pentadiene, 2-pentyne, and 1-methylcyclopropenyl methyl ketone (Scheme 7) (70JPC574) the ring contraction showed a high selectivity. 

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. 

In polar solvents the equilibrium (Scheme 6-43) is almost completely on the side of the open structure 6.63 a <- 6.63 b. Irradiation at A > 350 nm results in selective quantitative cleavage of the diazocyclohexadienone with loss of N2 and a ring-contraction to give a five-membered ring of the carbene intermediate, 6-fulvenone (6.65). 

Anew route to 2,5-dihydrofiirans and tetrahydro[3,2-b]furans via ring contraction of pyranoside C-glycosides was reported <96CC1663>. ( )-Homononactic acid and its 8-epimer were synthesized by using a c -selective iodoetherification as the key step (Scheme 21, <96SL777>). 

In summary, in order to reduce the content of toluene in gasoline while keeping a high octane number, toluene must undergo hydrogenation and ring contraction followed by SRO. The RC step can proceed via bifunctional catalysts and the SRO must use a metal catalyst (e.g. Ir/Si02) that is selective towards the dicarbene mechanism to cleave C-C bonds at unsubstituted positions. 

Selective silylation of aldohexono-1,4-lactones with tert-butylchlorodi-methylsilane gave the 2,6-di-O-silylated derivatives as major products (40). However, D-glucono-1,5-lactone underwent ring-contraction during silylation to give 2,6-di-O-terl-butyldimethylsilyl-D-glucono-1,4-lactone (31a) together with its 5,6-di-O-silyl isomer (31b), in 65 and 18% yields, respectively. 

Zeolite crystal size can be a critical performance parameter in case of reactions with intracrystalline diffusion limitations. Minimizing diffusion limitations is possible through use of nano-zeolites. However, it should be noted that, due to the high ratio of external to internal surface area nano-zeolites may enhance reactions that are catalyzed in the pore mouths relative to reactions for which the transition states are within the zeolite channels. A 1.0 (xm spherical zeolite crystal has an external surface area of approximately 3 m /g, no more than about 1% of the BET surface area typically measured for zeolites. However, if the crystal diameter were to be reduced to 0.1 (xm, then the external surface area becomes closer to about 10% of the BET surface area [41]. For example, the increased 1,2-DMCP 1,3-DMCP ratio observed with decreased crystallite size over bifunctional SAPO-11 catalyst during methylcyclohexane ring contraction was attributed to the increased role of the external surface in promoting non-shape selective reactions [65]. 

Vaughan, D.E.W., Strohmaier, K.C., Kliewer, W.R., and Leta, D.P. (2005) Methylcyclohexane ring-contraction a sensitive solid acidity and shape selectivity probe reaction. J. Phys. Chem. B, 109 (6), 2222-2226. 

The N-acyliminium ion cyclization of glutarimide 78c with terminal alkene does not show the usual selectivity in favor of ring contraction pyridobenzazepine 78d and the isomeric chloromethylquinolizidine are isolated in nearly equal amounts (10OL1696). 

An interesting synthetic approach to thietanes is the selective desulfurization of cyclic disulfides.The treatment of dithiolanes with a diethyl-aminophosphine results in a ring contraction to thietanes, (Eq. 19). This has been demonstrated with a-lipoic acid, a coenzyme with a dithiolane structure involved in the biological oxidation of pyruvic acid. The reaction is proposed to be initiated by the electrophilic attack of the phosphorus on the ring sulfur atom, resulting in the formation of an acyclic internal phosphonium salt, which by subsequent elimination of a phosphine sulfide, closes to the four-membered ring. °... 

Methods have been developed for selective alkylation at nitrogen in (459) and (460) (77JOC2551). Compound (460) also underwent a base-induced ring contraction analogous to the (448) to (453) conversion discussed earlier. 

Photochemically induced ring enlargements of alk-l-enylcyclobutanes to cyclohexenes have been reported only in a few select examples. A steroidal cyclohex-3-enone was formed in an equilibrium reaction from a 2-alk-l-enylcyclobutanone precursor.84 With a cyclohex-2-enone only the reverse reaction, i.e. ring contraction to give a cyclobutanonc, was observed.85 86 In a tricyclic system, a [1,3] shift occurred on sunlight irradiation to form a cyclohexene in 90% yield from an alk-l-enylcyclobutane.87... 

As mentioned earlier, [5 —> 3 + 2] ring contraction of a five-membered ring compound with an exocyclic double bond can lead to a three-membered ring either on the least-motion or on the non-least-motion path. In the most general case, three different products are possible one is formed directly on the former and two can be formed indirectly on the latter path. We have investigated the relevant question of selectivity in some detail (92TH1). 

Scheme 93 Selective ring contraction of 5-spirocyclopropane isoxazolidines mediated by acids...

A revised version of this synthesis has been subsequently developed that has a higher overall yield and is more easily scalable [42]. Reasoning that the yield and selectivity problems in the previous route were due to the use of ferf-butyl hypochlorite, the transformation of 11-epz-fischerindole G 132 into (-)-fischerindole I 133 was carried out in excellent yield by exposure of 132 to DDQ in the presence of water, presumably through the intermediate unsaturated imine 136. For the final ring contraction step, it was decided to replace the previously employed chlorohydroxylation by a hitherto unknown fluoro-hydroxylation, expecting that the increased hardness of fluorine over chlorine... 

---