Cyclopent oxazines

Bobowsky and Shavel found an interesting intramolecular reductive transacylation reaction, in which substituted cyclopent[e][l,3]oxazin-2-ones and l,3-perhydrobenzoxazin-2-ones (90) were formed (80JHC277). In the reactions of 4-(2 -oxocycloalkyl)-3,4-dihydro-3-methyl-2//-l,3-benzoxazin-2-ones 88 and potassium borohydride, the 2 -hydroxycycloalkyl products 89 obtained underwent intramolecular transacylation reactions, resulting in the dihydro-1,3-oxazine derivatives 90. In this way, the 4-(2 -oxocycloalkyl)... 

Depending on the cis or trans configuration and the A -substituents, marked differences were found in the cyclizations of the corresponding ureas or thioureas to cyclopent[e][l,3]oxazines and octahydro-2//-l,3-benzoxazines (85T5981), when the cyclization occurs on an sp carbon. 

Only two articles deal with the synthesis of saturated 1,3-benzoxazine-2,4-diones and the corresponding cyclopent[e][l,3]oxazines. 

New types of fluorinated cyclopent[d]-3,l-oxazines and tetrahydro-4//-3,l-benzoxazines 180 and 181 were prepared (88IZV132 89TL-1987 91IZV1130, 91T5577). Cyclopentene, 1-methylcyclopentene, or 1,3-cyclohexadiene was reacted with a mixture of acetyl boron trifluoride and acetonitrile, resulting in the oxazines 180 and 181 regio- and stereospecifi-cally via acylamidation. 

Fusion of the 3-aryl-cyclopent[e][l,2]oxazines (374) at 200—280 °C gives 2-aryl-pyridines (375) in 40—80% yield a plausible mechanism is depicted in Scheme 145. Acid-catalysed ring-opening takes a quite different course, ring-contraction occurring (Scheme 146). ... 

The first examples of the ten-electron cyclopent[rf][l,3]oxazine system, e.g. (388), have been synthesized, in moderate yields, from cyclohexa-2,4-dienimines (386), as exemplified in Scheme 150. The intermediate ketenimines (387) can be observed spectroscopically [p = 2020 cm ) if the reaction takes place at low temperature (77 K). ... 

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