Oxaspiro rearrangement

Oxo-4-oxaspiro[2.3]hexanes are rearranged in the presence of a copper catalyst to give 2(5H)-furanones and 2(3/f)-furanones [94]. 

The cyclization of 1-cyclohexeneacetic acid (1) has been studied under a variety of conditions. Treatment of the thallium salt with iodine affords a mixture of the y-iodo-/ -lactone 3 and a /J-iodo-y-lactone 29. (47 , 55 )-5-Iodo-l-oxaspiro[3.5]nonan-2-one (3), which predominates in the mixture, rearranges to the thermodynamically more stable /i-iodo-y-lactone. nV-hexahydro-3a-iodobenzofuran-2(3/7)-one (2). 

Transannular hydride shifts, first detected by Cope and coworkers in solvolyses of cyclooctene oxide, have subsequently been found in a number of related systems, e.g. cyclooctadiene monoepoxides, CA o-bicyclo[3.3.1 ]non-2-ene epoxide and l-oxaspiro[2.6]nonane. In general these reactions do not involve skeletal rearrangements, and they will not be discussed in detail. 

Rearrangements of oxaspiro compounds such as l,5-dioxaspiro-(2,6) octane and... 

Tetramethyl-l-oxaspiro-(2,5)octane is conventionally obtained in a multi-step synthesis from readily available (-)-(5)- -citronellol or (H-)-(/ )-pulegone (Figure 8) [41], In the presence of acidic catalysts it rearranges to the industrially-desired 2,2,3,6-tetramethylcyclohexane carbaldehyde (17) and many other products. 

Phosphoryl-substituted 2-vinylcyclopropanes have been prepared by the addition of photochemically generated carbenes to 2,3-dimethylbutadiene." Similarly, benzophenone-sensitized photodecomposition of the dimethyl (a-diazoalkyl)phosphonates (68) in the presence of diketen (69) affords E- and Z-1-substituted l-(dimethylphosphono)-5-oxaspiro[2.3]hexanes (70) and (71). Rearrangement of the carbene (72), leading to the formation of short-lived... 

The vinylcyclopropane rearrangement is an important part of Trost s seco-alkylation sequence (see sec. 8.8.B.ii). In this protocol, trimethylsilyloxyvinylcyclopropanes such as 524 are generated by reaction with diphenylcyclopropyl sulfide and subsequent reaction with lithium diisopropyl amide (LDA), to open the oxaspiro-pentane) and trapping the resulting alkoxide anion with chlorotrimethylsilane. Thermal... 

A similar mechanism is involved in the boron-trifluoride-catalysed rearrangements of l-oxaspiro[2,3]hexanes to cyclopropanols and/or methyleneoxacyclopentanes (Scheme 38). 2s... 

The reaction of geminal dibromides and carbonyl compounds in the presence of butyl-lithium or lithium metal can provide a general route to a wide range of oxirans. Stevens and Pillai have used a Darzens cyclization reaction to form the first example of an isolable epoxy-amine (121). 2-(l-Aziridinyl)-2-phenyl-l-oxaspiro[2,5]octane (121) was relatively stable and rearranged to the corresponding cycloheptanone (122) only after... 

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