Pyrolysis cycloelimination

Sol 7. In amine oxide pyrolysis, cycloelimination uses a ring of five atoms, therefore, the reaction is necessarily syn selective. The oxide of trans-l-phenyl-2-dimethylaminocyclohexane undergoes syn elimination to give... 

Sol 2. (i) In selenoxide, pyrolysis cycloelimination uses a ring of five atoms therefore, the reaction is necessarily yu-selective. Hence, on heating, cyclo-hexylphenyl selenoxide undergoes iyn-elimination regioselectively to produce the less stable Hofmann product. 

An even milder cycloelimination uses a ring of five atoms 6.28 instead of six, but still involves six electrons. This is no longer a retro-ene reaction, but it is still a retro group transfer and it is allowed in the all-suprafacial mode 6.29. The pyrolysis of N-oxides 6.30 is called the Cope elimination, and typically takes place at 120°, the corresponding elimination of sulfoxides 6.31 (X=S) typically takes place at 80°, and, even easier, the elimination of selenoxides takes place at room temperature or below. All these reactions are affected by functionality making these numbers only rough guides, but they are all reliably syn stereospecific. 

Diaminocyclopropane derivatives (528) were cleaved upon vacuum flash pyrolysis. Besides cycloalkene (529), amidines (531) were obtained in 7 91 % yield. Compounds 531 were believed to be the result of a ring enlargement of the initially formed diaminocarbenes (530) (equation 138). Pyrolysis of cis- and rrans-bicyclododecane derivatives occurred with distinct stereoselectivity during cycloelimination (ratio of cis- rrans-cycloundecene 93 7 and 13 87 respectively). 

Flash-vacuum pyrolysis of bis(dialkylamino)cyclopropanes can cause a nonoxidatively induced cycloelimination to give cycloalkenes and amidines. Thus, decomposition of 7,7-dimor-pholinobicyclo[4.1.0]heptane (8) at 700°C (10 Torr) gives heterocyele 9 and cyelohexene in good yield. The corresponding pyrrolidino and piperidino compounds are similarly cleaved to give 2,3,4,5-tetrahydro-6-pyrrolidinopyridine and 3,4,5,6-tetrahydro-7-piperidino-2//-azepine, respectively. The intermediacy of diaminocarbenes has been proposed for the formation of these heterocycles. 

Another attractive feature of the phosphonamide route is that it can be controlled to give cis- and trans-olefins.4 Formation of the adduct is not stereospecific, but the elimination reaction is stereospecific (cis cycloelimination). The synthesis of cis- and trans-1 -phenylpropene is illustrative the former was prepared from the reaction of benzaldehyde with the a-lithio derivative of ethylphosphonic acid bis(dimethyl-amide) to give two diastereoisomeric /3-hydroxyphosphonamides (5) in the ratio of 3.5 1. The major isomer was separated by crystallization and on pyrolysis gave cis-1 -phenylpropene. The isomeric olefin was prepared by reaction of the a-lithio... 

The original method for the synthesis of triflnoromethyl-snbstitnted oxazoles is based on the transformation of acyloxazolin-5-ones. Thus, 2-acyl-2-trifluoromethyl-A -oxazolin-5-ones 38 undergo cycloelimination of CO2 on heating at 200-230 °C to yield trisubstituted oxazoles 39. In this case snbstituents at positions 2 and 4 of the oxazole ring are formally interchanged [37]. Flash vacnnm pyrolysis (540-600 °C/0.01 mm Hg) of N-trifluoroacetylisoxazole-5-one also leads to 2-trifiuoro-methyloxazole [38]. 

Cyclic Orthoformates. When the above strategy is applied to polyols, cyclic orthoformates can be isolated. Most common are the cyclization of 1,3-diols (eq 2) and 1,2-diols, as well as the formation of caged structures from the use of polyols (eq 3). Those orthoformates obtained from 1,2-diols (eq 4) can undergo cycloelimination upon pyrolysis to afford alkenes in high yield. There are a variety of methods for carrying out this overall... 

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