2,3,4,8,9,9a-Hexahydropyrido

Dialkyl 7-aryl-6,9a-dimethyl-2,3,4,8,9,9a-hexahydropyrido[2,l-h][l,3]-oxazine-7,9-dicarboxylates exhibit long-term antihypertensive-brady-cardic, anti-inflammatory, and spasmolitic effects (92MI1). 

Intramolecular Diels-Alder reactions of azadienes 76 afforded 6-cyano-2,3,4,8,9,9a-hexahydropyrido[2,l-6][l,3]oxazines 77 (99TL7211, 99TL7215). Cyclization was carried out smoothly at or below room temperature in... 

A diastereoselective intramolecular hetero-Diels-Alder reaction of optically active 428 gave unstable 1,3,4,8,9,9a-hexahydropyrido[2,l-f][l,3]oxazin-l-one 429 (X = 0), and l,3,4,8,9,9a-hexahydropyrido[l,2-(z]pyrazin-l-one 429 (X = NTs) (Equation 80) <2003JA4970, 2004T10277>. In the case of pyrido[l,2- ]pyrazine, the reaction was carried out in the presence of 2,6-di- /-butyl-4-methylphenol. 

Treatment of 2-methyl-9a-methoxy-4,6,7,8,9,9a-hexahydropyrido[2,l-b][l,3]oxazin-4-one with cone, ammonium hydroxide in a sealed tube gave 2-methyl-6,7,8,9-tetrahydro-4//-pyrido[l, 2-a]pyrimidin-4-one [75H(3)927]. 

Attempts to reduce the quaternary salts of 4-oxo-4//-pyrido[l,2-a]-pyrimidines with sodium borohydride or lithium aluminum hydride remained unsuccessful.137 At the same time the 6,7,8,9-tetrahydro quaternary salts may readily be reduced with sodium borohydride to the 1-alkyl-l,6,7,8,9,9a-hexahydro derivatives.75-77,133 1481269 270 Sodium borohydride reduction of the 1,6- and l,7-dimethyl-3-carbamoyl-4-oxo-6,7,8,9-tetrahydro-4H-pyrido[l,2-a]pyrimidinium salts proceeds stereoselectively and yields the thermodynamically controlled product.271 The l-methyl-3-carbamoyl-4-oxo-l,6,7,8,9,9a-hexahydropyrido[l,2-a]pyrimidines were also prepared by the catalytic (Pd/C) hydrogenation of the 1,6,7,8-tetrahydro derivatives,270-272 but this reaction led to a diastereoisomeric mixture.271... 

Hydrogenation of 6-methyI-4-oxo-4//-1,6,7,8,9,9a-hexahydropyrido[ 1,2-a]pyrimidine-3-carboxylic ester to the perhydro derivative was performed by catalytic (Pd/C) hydrogenation. The 1-methyl derivative was reduced by sodium borohydride.75133... 

Irradiation of piperidine 62 in CH2CI2 at concentration of 3mg/ml in the presence of 1-methylimidazole using a high-pressure mercury lamp (150 w) yielded 2-phenyl-4,6,7,8,9,9a-hexahydropyrido[2,l-6][l,3]oxazin-4-one (01S1258). 

---