Spiropentanes

In contrast to the small ring bicyclic hydrocarbons that resulted in successful outcomes, the reactivities of spirocycUc strained hydrocarbons with transition metals have been less studied. 

Spiro [2.2] pentane is stable to AgBF in CHClg [43]. Hydrogenation of spiropentane occurred in the presence of Pt02 to give a mixture of neopentane, 1,1-dimethylcyclopentane, and isopentane via C-C bond cleavage of spiropentane [44]. 

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Organic molecules m which one carbon atom is common to two rings are called spirocyclic compounds The simplest spirocyclic hydrocarbon is spiropentane a product... 

A short-path distillation apparatus is used, the distillate (oxa-spiropentane plus dichloromethane) being trapped in a reeeiver placed in a methanol-dry ice bath cooled to — 80°. The checkers found it useful to drive out last traces of product by adding several milliliters of dichloromethane to the residual thick paste and distilling. The proton magnetic resonance spectrum (dichloromethane) shows an oetet at 8 0.85 and a singlet at S 3.00 in the ratio 4 2. 

A second mole gives spiropentanes. In fact, any size ring with an exocyclic double bond can be converted by a carbene to a spiro compound. ... 

Scheme 34 SE (in parenthesis in kcal mol" ) of spiropentane and spiropentasilane derivatives...

In a study aimed at elucidating the mechanism of the thermal decomposition of spiropentane 229, the two regioisomeric pyrazolines 227 and 228 were obtained in high yield by allowing a solution of MCP (1) and diazomethane (226) (or diazomethane-d2) in diethylether to stand at 3 °C for three weeks (Scheme 37) [59]. 

The earlier examples of [2 + 1] cycloaddition of a carbene (or carbenoid) on the double bond of alkylidenecyelopropanes to yield spiropentane derivatives were observed as undesired side reactions in the synthesis of alkylidenecyelopropanes through the addition of a carbene to a substituted allene [161]. In some cases the spiropentane derivative was obtained as the major product [161a, c] especially when a large excess of the carbene reagent was used. For example, when methyl 3,4-pentadienoate (610) was treated with a ten-fold excess of methylene iodide and zinc-copper couple the two products 611 and 612 were isolated in 1 4.5 ratio (Scheme 86) [161a]. 

Diazomethane in the presence of palladium acetate gave with allenes 613 a similar mixture of methylenecyclopropanes 614 and spiropentanes 615. In contrast to Simmons-Smith reagent, diazomethane prefers to add to the less substituted allenic double bond (Scheme 87) [162]. 

The cyclopropanation of a-allenic alcohols 616 gave methylenecyclop-ropanes 617a, b and spiropentanes 618a, b in different proportions depending on the carbenoid reagent used (Scheme 88) [163,4b]. 

The use of an excess (3 equivalents) of the Simmons-Smith reagent gave exclusively the spiropentane derivatives 618 in 60-70% yield [163a], In contrast, not even trace amounts of these products were observed by using samar-ium-dihalomethane as the carbene source [4b],... 

The use of carbene additions to MCP derivatives has lately become a general strategy for obtaining spiropentanes and higher spiranic triangulanes. 

The rhodium acetate catalyzed addition of ethyl diazoacetate to MCP (1) gave spiropentane 619 in high yield (Scheme 89) [6e]. The same compound 619 was obtained in lower yield by a Simmons-Smith reaction to methylenecyclo-propane 217 [164],... 

Thermally generated perchlorovinyl carbene 623 reacted with MCP (1) to give the spiropentane derivative 624 in modest yield (Scheme 93) [167],... 

Several optically active spiropentanes were obtained by carbene addition to chiral methyleneeyclopropanes (Table 48) [168,169],... 

An asymmetric synthesis of the spiropentanes 630, albeit with low enantiomeric excess, was achieved by the reaction of allenes 629 with diazomethane in the presence of an optically pure copper (II) chelate complex (R) or (S)-631 (Scheme 94) [170],... 

The most remarkable feature of the chemistry of these ylides is their efficient participation in typical sulfur ylide chemistry, i.e. their ability to form epoxides with carbonyl partners (Eq. 27 a)29 and spiropentanes with enones (Eq. 27 b)30. ... 

One such typical transformation is the thermal isomerization of the spiropentane derivative 76 into triene 80 which is assumed to occur via the diene intermediate 78 with the intermediate participation of the cyclopropyl-trimethylenemethane (TMM) 77 and the vinyl-TMM 79 diradicals (equation 29)44. It was shown by using deuterium labels that the diradical 79 forms the triene 80 by 1,6-hydrogen shift. The pathway 76 — 80 which occurs via tetramethylene-ethane diradical was recognized as a less probable route. 

Vinylbuta-1,3-diene produces the 1,2- and 3,4-mono-insertion adducts with dichlorocarbene in a 4 1 ratio [37]. A similar preference in reactivity is observed with 3-methylene-cyclohexenes [90]. 1,2-Dienes react with two equivalents of dichlorocarbene to form spiropentanes [21, 90] (Scheme 7.6). Spiropentanes (50-95%) are also obtained from methylenecyclopropanes [36,106] and by the reaction of electron-deficient alkenes with an excess of chloroform [31] (see Scheme 7.12)... 

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