1.3- Oxazines, 5,6-dihydro— from

A formation of 5,6-dihydro-l,3-477-oxazine derivatives from 3-acetylaminopropanol derivatives has also been described " " - (Scheme 2). 

The ring opening of tetrahydro-l,3-oxazines to aldehydes has recently found wide application through the work of Meyers.2-3 2-Alkylidene-tetrahydro-l,3-oxazines, prepared from the readily available 5,6-dihydro-4//-1,3-oxazines, possess strong nucleophilic properties and can react with alkyl halides and carbonyl compounds. The derivatives so obtained can be reduced to tetrahydro-l,3-oxazines, and through ring opening the latter can furnish acyclic, alicyclic, and a,jS-unsaturated aldehydes and their C-l deuterated derivatives.221-223 228... 

Circularly polarised 190 nm radiation has been used to study the direct photoracemisation of the enantiomers of trans cyclo-octene (Inoue et al.), and new evidence has been reported for the 90° twisted intermediate on the SI surface of stilbene (Gano et al.). Intramolecular charge-transfer excitation of 4-dimethyl-amino-4 -cyanostilbene and 4-azetidinyl-4 -cyanostilbene is considered to involve at most an intermediate with a lifetime of less than 1 ps (lUchev et al.), and a novel synthetic route to 5,6-dihydro-4H-l,2-oxazines (26) from y,5-unsaturated oximes has been described by Armesto et al. The photorearrangement of the ammonium salts of the dibenzobarrelene (27) to the semibullvalene isomers has... 

The products are 5,6-dihydro-2il-1, 3-oxazines (128) from predominantly endo reaction and yields are high (70-95%). 

Potassium carbonate 5,6-Dihydro-l,2-oxazine ring from 4,5-dihydro-l,2-oxazinium ring... 

Karrer thought that the dihydro-1,3-oxazine skeleton can possibly be formed in proteins during their transformation to polypeptides. Fodor - and Fiescr explained the stereospecific migration of acyl groups from N to 0 and vice versa in tropine alkaloids by an intermediate formation of a 1,3-oxazine ring. 

Dihydro-l,3-4H-oxazines (33) can be formed from 0-acyl derivatives of 3-propanolamine even where the formation of smaller rings would also have been possible/ e.g. ... 

The only known derivative of this class was prepared from ethyl 8-anilinecrotonate, ethyl acetoacetate, and benzaldehyde. In the first instance, a pyrimidine derivative is formed, this is then subjected to partial hydrolysis to form the 3,4-dihydro-l,3-2H-oxazine derivative (42). 

When an S-alkyl-A A -disubstituted isothiourea reacts with diketene in a boiling solvent, 2-imino-4-keto-3,4-dihydro-l,3-2i/-oxazine derivatives (45) are formed. Analogous compounds can be prepared from dialkylcarbodiimides and diketene. ... 

N-Substituted 5,6-dihydro-2//-1,2-oxazines were found to be significantly more stable than their N-unsubstituted analogs and could be distinguished from the corresponding 4H isomers using H NMR spectroscopy. Thus, it was shown that oxazinium salt 80 isomerizes on treatment with sodium carbonate to tricyclic... 

We are the first group to succeed with the highly enantioselective 1,3-dipolar cycloadditions of nitronates [75]. Thus, the reaction of 5,6-dihydro-4H-l,2-oxazine N-oxide as a cyclic nitronate to 3-acryloyl-2-oxazilidinone, at -40 °C in dichloro-methane in the presence of MS 4 A and l ,J -DBFOX/Ph-Ni(II) complexes, gave a diastereomeric mixture of perhydroisoxazolo[2,3-fe][l,2]oxazines as the ring-fused isoxazolidines in high yields. The J ,J -DBFOX/Ph aqua complex prepared from... 

A synthesis of aldehydes developed by Meyers begins with the commercially available dihydro-1,3-oxazine derivatives 132 (A = H, Ph, or COOEt). Though the ions (133) prepared from 132 are ambident, they are regioselectively alkylated at... 

Tetrahydropyrido[2,1 -HI 1,4 oxazinc-7,9-dicarboxylate 278 was obtained from the 1,8-dihydro derivative 277 by hydrogenation over a Pd/C catalyst (Equation 51) <1997CAP2188071 >. Catalytic hydrogenation of an epimeric mixture of (4.S, 9a.S )-l -trimcthylsilyloxy-4-phenyl-3,4,6,7-tetrahydro-l//-pyrido[2,1 -HIl,4]oxazine over Raney-Ni afforded perhydro derivatives <2000SC2565>. 

Reduction of the Oximino Fragment in Substituted 5,6-Dihydro-4H-Oxazines Catalytic hydrogenation of substituted dihydro-477-oxazines (552), as well as their reduction with sodium cyanoborohydride (553), were studied in sufficient detail and were used in several total syntheses. However, the use of silylation of six-membered cyclic nitronates enables the synthesis of previously unknown dihydrooxazines containing functionalized substituents at the C-3 and C-4 atoms from easily available precursors. 

An intermediate 5-hydroxy-5,6-dihydro-2/7-pyrrolo[l,2- ][l,2]oxazin-7(4a//)-one 142 has been described in the total synthesis of (—)-loline, a pyrrolizidine alkaloid extracted from rye grass Lolium cuneatum. The key step of the synthesis was an intramolecular cycloaddition of acylnitrosodienes (obtained by in situ oxidation of the corresponding hydroxamic acids 143). This reaction generated predominantly the rro/o-stereoisomer that was further cleaved at the N-O bond with Na(Hg) and further elaborated in several steps to reach the target compound (Scheme 19) <2001J(P 1)1831 >. 

ALDEHYDES FROM ALLYLIC ALCOHOLS AND PHENYLPALLADIUM ACETATE 2-METHYL- 3-PHENYLPROPIONAL-DEHYDE, 51, 17 ALDEHYDES FROM AROMATIC NITRILES p-FORMYLBENZENE-SULFONAMIDE, 51, 20 ALDEHYDES FROM 2-BENZYL-4,4,6-TRIMETHYL—5,6-DIHYDRO-l, 3-(4H)-OXAZINE 1-PHENYLCYCLO-PENTANECARBOXYALDEHYDE, 51,... 

Aldehydes, a-phenyl-, from 2-benzy1-4,4,6-trimethyl-5,6-dihydro-1, 3(4H)-oxazine,... 

In qualitative terms, the rearrangement reaction is considerably more efficient for the oxime acetate 107b than for the oxime ether 107a. As a result, the photochemical reactivity of the oxime acetates 109 and 110 was probed. Irradiation of 109 for 3 hr, under the same conditions used for 107, affords the cyclopropane 111 (25%) as a 1 2 mixture of Z.E isomers. Likewise, DCA-sensitized irradiation of 110 for 1 hr yields the cyclopropane derivative 112 (16%) and the dihydroisoxazole 113 (18%). It is unclear at this point how 113 arises in the SET-sensitized reaction of 110. However, this cyclization process is similar to that observed in our studies of the DCA-sensitized reaction of the 7,8-unsaturated oximes 114, which affords the 5,6-dihydro-4//-l,2-oxazines 115 [68]. A possible mechanism to justify the formation of 113 could involve intramolecular electrophilic addition to the alkene unit in 116 of the oxygen from the oxime localized radical-cation, followed by transfer of an acyl cation to any of the radical-anions present in the reaction medium. 

Optically pure 2,3-dihydro-6//-l, 4-oxazin-2-ones were prepared in a two-step reaction from A-protected AAs and a-bromoketones (78JOC135). When heated in boiling toluene and HCl 2,2-dimethylamino acid amides were transformed into the corresponding 3,3-dimethyl-l,4-oxazine-2,5-diones (83TL1921 87HCA329). 

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