1.2- Oxazine ring opening

Early picosecond studies were carried out by Schneider et al, [63] on the parent spiro-oxazine (NOSH in Scheme 8) and similar derivatives. In a back-to-back work, they also described a complimentary CARS (coherent anti-Stokes Raman spectroscopy) investigation [69], Simply put, these authors found that the closed spiro-oxazine ring opened in 2-12 psec after laser excitation. The reaction was slower in more viscous solvents. An intermediate state formed within the excitation pulse and preceded the formation of merocyanine forms. This transient was named X in deference to the X transient named by Heiligman-Rim et al. for the spiropyran primary photoproduct [8], (See also the previous section.) The name X has since been adopted by other workers for the spiro-oxazines [26,65],... 

Oxazine ring opening with oxazino[2,3-d]-pyrimidines... 

Oxazine ring opening of the sugar spiro derivative 699 with phenylhydra-zine gave the 1,2,4-triazole C-nucleoside 702 through the intermediates shown in Scheme 187 (85CPB102),... 

Katagiri et al. prepared the 2-pyriniidyl C-nucleoside 912 by two routes (85CPB102) the first involved reaction of the 2,5-anhydro-D-allonic acid derivative 65 with 3-aminocrotonamide followed by cyclization and removal of the protective groups. The second route comprised oxazine ring opening of the Spiro compound 699 with amonia. Riley obtained the same C-... 

Platinum oxide/hydrogen chloride Reductive tetrahydro-l,2-oxazine ring opening... 

A common feature of tetrahydro-l,3-oxazines is their ability to be hydrolyzed with ring opening. This occurs particularly readily in the presence of dilute mineral acids—preferably methanolic or ethan-oiic,26,27,58,158,159 pjjg procBss Can be much speeded up and the yield of... 

Ring opening of some derivatives of tetrahydro-1,3-oxazine can also occur when warmed with aqueous sodium hydroxide ... 

The simple ring opening of tetrahydro-1,3-oxazine derivatives is not the only possible reaction of these heterocyclic compounds catalyzed by mineral acids. An interesting rearrangement of 6-aryl-6-alkyltetrahydro-l,3-oxazines when warmed with concentrated hydrochloric acid was found by Schmiedle and Mansfield ... 

Gabriel has demonstrated the instability of 5,6-dihydro-1,3-4/f-oxazines by reacting the hydrobromide of 2-phenyl-5,6-dihydro-l,3-4i -oxazine (55) with water. Ring opening occurs with the formation of 3-aminopropylbenzoate which is rearranged into 3-benzamido-propanol. 

King and Durst have recently found the ring opening of trisubsti-tuted isoxazole quaternary salts to result in the formation of 1,3-(2 )-oxazine derivatives. ... 

Reductive N-O bond cleavage of perhydropyrido[l,2-6][l,2]oxazine 10 with Zn dust furnished 2-(3-hydroxypentyl)piperidine 11 (96JCS(P1)1113). Similarly, 2/S,4u S,5Q ,7/0,8yS-H-5-benzyloxy-7-(tert-butyldiphenylsilyloxy)-2-[2-(methoxymethoxy)ethyl]-8-methylperhydropyrido[l,2-6][l,2]oxazine gave the respective ring-opened piperidine (OOOL2955, 01JOC3338). 

Reaction of 8-methylperhydropyrido[l,2-c][l,3]oxazine-l,3-dione 91 with PhCH2NH2 then PhCOCl and 4-methoxyphenol afforded ring-opened products 92 and 93, respectively (00JA11009). 

Treatment of perhydropyrido[2,l-c][l,4]oxazine-3,6-dione 232 with B2H6 yielded (—)-(2i )-[(2S)-hydroxymethyl)piperldin-1 -yl]-2-phenylethanol (233) (00T233). Reduction of ( )-(3i ,4i ,9aS)-4-methyl-3-phenylperhydropyr-ido[2,l-c][l,4]oxazin-3-ol (234) with NaBH4 yielded ring-opened product 235 (97JHC1813). 

Treatment of perhydropyrido[2,l-c][l,4]oxazin-l-ones 236 with vinyl and 1 -chloroethyl chloroformate in the presence of 4A molecular sieves afforded ring-opened (2i )-pipecolic acid derivatives 237 (97SL799, 98EJOC2461, 98T10309). Similarly, perhydropyrido[l,2-c][l,4]oxazin-l-one 238 yielded (2S)-pipecolic acid derivative 239 (98T10309). 

The relative rate of cationic homopolymerization is governed by three factors, ie. the concentration of the propagating species, the ring-opening reactivity of the growing species and the nucleophilic reactivity of the monomer. From kinetic studies196 197 of the polymerization of oxazolines and oxazines it was found that the second factor was the most important. On the other hand, the relative reactivity in the cationic copolymerization is mainly determined by the nucleophilicity of the monomer and for 2-substituted 2-oxazolines this is in the order of benzyl > methyl > > isopropyl > H > phenyl195. ... 

Allylpiperidines were formed from 3-iodomethylperhydropyrido[l,2-f][l,3]oxazin-l-ones by treatment with Zn in AcOH <2002OL3459>. l-Methyl-2-(2-hydroxyalkyl)piperidines were prepared from 3-substituted perhydropyr-ido[l,2-tf][l,3]oxazin-l-ones with lithium aluminium hydride (LAH) in boiling THF and EtzO <2005T1595>. Reaction of 8-methylperhydropyrido[ 1,2-r [ 1.3 ]oxazine-1,3-dione 103 with PhCH2NH2, then PhCOCl and 4-meth-oxyphenol afforded ring-opened products 104 and 105, respectively (Scheme 8) <2000JA11009>. 

Treatment of perhydropyrido[2,T2][l>4]oxazin-l-ones 259 with vinyl and 1-chloroethyl chloroformate afforded ring-opened (2A)-pipecolic acid derivatives 260 (Equation 49) <1997SL799, 1998EJ02461, 1998T10309>. Similarly, perhydropyrido[l,2-f][l,4]oxazin-l-one 261 yielded (26 )-pipecolic acid derivative 262 (Equation 50) <1998T10309>. 

The pyrimido[2,T ][l>3]oxazines 301 are sensitive to water and suffer reversible ring opening to the lactams 302. The equilibrium favors the ring-opened product (Equation 31) <20050BC1964>. 

Hydrolytic ring opening of the 1,3-oxazine ring of the tetrahydro-[l,3]oxazino[3,4- ][l,2]oxazin-8-one 389 and subsequent protection of the carboxyl group has been performed (Scheme 61) <1999TL4391>. 

It is very likely, that this reaction occurs due to the equilibrium between trimethylsilyl halide and a nitrogen-containing nucleophile, which increases the electrophilicity of silyl Lewis acids. It should be noted that the configuration of stereocenters at the carbon atoms of the oxazine ring is partially distorted. Hence, it is assumed that the reaction proceeds through the intermediate cation B, which is partially isomerized into the stereoisomeric cation B through the open chain cation B". 

Enantiomerically pure tetrahydro-l//-pyrrolo[2, l -acrylamides derived from proline (see Section 11.11.7.4), are versatile intermediates for the synthesis of natural products or drugs. Compound 86a was submitted to debromination with Bu3SnH followed by ring opening in KOH and further reduction with BHj to give diol 89 that was then easily transformed into (A)-4-(2,2,4-trimethyl-l,3-dioxolan-4-yl)-lT>utanol 90, a key intermediate for )-frontalin, <2002TA155>,... 

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