Extended system ring opening

The asymmetric oxidation of organic compounds, especially the epoxidation, dihydroxylation, aminohydroxylation, aziridination, and related reactions have been extensively studied and found widespread applications in the asymmetric synthesis of many important compounds. Like many other asymmetric reactions discussed in other chapters of this book, oxidation systems have been developed and extended steadily over the years in order to attain high stereoselectivity. This chapter on oxidation is organized into several key topics. The first section covers the formation of epoxides from allylic alcohols or their derivatives and the corresponding ring-opening reactions of the thus formed 2,3-epoxy alcohols. The second part deals with dihydroxylation reactions, which can provide diols from olefins. The third section delineates the recently discovered aminohydroxylation of olefins. The fourth topic involves the oxidation of unfunc-tionalized olefins. The chapter ends with a discussion of the oxidation of eno-lates and asymmetric aziridination reactions. 

Tautens has extended this chemistry to include the alkylative ring opening of these systems using aryl and vinyl boronic acids as nucleophiles in the presence... 

In the present study, the novel concept of the Lewis acid assisted living polymerization with the aluminum porphyrin-methylaluminum diphenolate (3) system was successfully extended from the accelerated living addition polymerization of alkyl methacrylates to the accelerated living ring-opening polymerizations of lactones. 

The only truly stable derivative of this system, 3,4-bis(trifluoromethyl)-l,2-dithietene, has a characteristic IR absorption at 1629 cm 1 and UV Amax of 238 (e =7440) and 340 nm (e =80) (60JA1515). An extended Hiickel calculation shows the favored isomer to be the ring-opened tautomer, dithioglyoxal, while the CNDO/2 method favors the ring-closed 1,2-dithietene tautomer (75JCS(P2)559). 

Functionalized cyclopropenes are viable synthetic intermediates whose applications [99.100] extend to a wide variety of carbocyclic and heterocyclic systems. However, advances in the synthesis of cyclopropenes, particularly through Rh(II) carboxylate—catalyzed decomposition of diazo esters in the presence of alkynes [100-102], has made available an array of stable 3-cyclopropenecarboxylate esters. Previously, copper catalysts provided low to moderate yields of cyclopropenes in reactions of diazo esters with disubstituted acetylenes [103], but the higher temperatures required for these carbenoid reactions often led to thermal or catalytic ring opening and products derived from vinylcarbene intermediates (104-107). 

Several norcarene derivatives were shown to undergo dehydrogenation with ring opening to form the more extended % system of cycloheptatriene. Thus, irradiation of chloranil in the presence of tricyclo[4.4.1.016]undeca-3,8-diene (45) gave rise to bicyclo[4.4.1]undeca-l,3,6-8-tetraene (46), while the quinone was reduced to the corresponding hydroquinone [229, 230]. 

The palladium-catalyzed system can be extended to the acylation of siloxycyclopropanes with aroyl chloride/carbon monoxide or aryl triflate/carbon monoxide, which gives 1,4-diketones. Contrary to the case of doubly oxygen-substituted cyclopropanes vide infra), the acylation of 1-siloxycyclopropanes is restricted to aroyl chlorides and is not applicable to aliphatic or a, -unsaturated acyl chlorides. For the reactions with aryl triflates, tetrakis(triphenylphos-phane)palladium(O) is used as catalyst, while the reactions with aroyl chlorides employ bis(triphenylphosphane)palladium(II) chloride and ( / -allyl)chloropalladium dimer/triphenyl phosphite as catalysts. In these reactions, aroylpalladium(II) species may undergo ring opening of the siloxycyclopropanes. 

A detailed examination of the preparation and photochemical activity of strained paracyclophanes has been reported. The starting materials are the Dewar benzenes (94). Irradiation of these at 254 nm at 77 K results in ring opening to the corresponding paracyclophane. Interestingly the unsubstituted derivative (94, R = H) breaks down to yield (95) and ethene on prolonged irradiation. This reactivity is not observed for the substituted compounds. The work was extended to cover systems such as (96) and (97), which both follow the... 

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