1.3- Thiazine skeleton

An asymmetric route to the 5,6-dehydro-4H-1,3-thiazine skeleton via an asymmetric /refero-Diels-Alder reaction [95]... 

Condensation of thioamides or thiourea with, -unsaturated ketones is a general method for the construction of the 1,3-thiazine skeleton . -Chlorovinyl ketones 343 and the thioamide 344 in the presence of perchloric acid give intermediate thioimidium salts 345, which cyclize to yield 1,3-thiazinium salts 346 (Scheme 160) <1982T937>. 

The di- or tettahydto-l,4-thiazine skeleton is obtained if mercaptans which have a carbonyl group in the P-position react with ethyleneimine (120—124)... 

In these syntheses, the linear heteroatomic chain must contain the six atoms of the future thiazine skeleton. The chain may be cyclized between N and C-4 (type 4a), between N and C-2 (type 4b), between S and C-2 (type 4c), between S and C-6 (type 4d), or finally between C-4 and C-5 (type 4e). 

Guingant has shown that asymmetric hetero Diels-Alder cycloadditions could be carried out under high pressure [63]. He used an as5unmetric route to the 5,6-dihydro-4H-l,3-thiazine skeleton via an asymmetric hetero Diels-Alder reaction (Scheme 42). Without catalyst, product 174 is formed preferentially, while the addition of Lewis acid catalyst reverts to enantioselective formation of product 173. When cycloaddition is performed at atmospheric pressure (dichlo-romethane, 110 °C, 20h, 75%), ratio of products is almost identical to high-pressure reaction (20 80), and with the addition of MgCl2 catalyst (at 0°C, 3h) the ratio 173 174 of 100 0 shows the same enantioselectivity as high-pressure conditions. 

The perhydropyrido[2,l-b][l,3]oxazine skeleton is a constituent part of macrocyclic xestospongine/araguspongine and aragupetrosine alkaloids isolated from different marine sponges. Pyrido[2,l-fo][l,3]oxazines, pyrido[2,l-fc][l,3]thiazines, and [l,3]oxazino[3,4-a]quinolines are also applied as key intermediates in the total syntheses of different alkaloids. Other examples of these ring systems have aroused much interest owing to their valuable pharmacological properties. 

Most of the methods described during the period covered by this chapter use pyridine intermediates and the skeletons of the oxazine and thiazine rings are built onto the pyridine ring. 

Loss of the Thiazine Ring a. No Change of the Ring Skeleton... 

A number of rearrangements are known in which new dihydrothiazines are formed from existing derivatives. One example, in which the ring skeleton remains unaltered, is provided by the conversion of compound 169 into 172 in the presence of lithium aluminum hydride presumably the intermediate dihydrothiazinol 170 undergoes dehydration to the thiazine 171, which cyclizes to the product. 

In spite of the above mentioned limitations, it should be noted that a lot of carbon-carbon and carbon-heteroatom coupling reactions have been summarized in this review. These reactions have given rise to about 200 new compounds in recent years. Complexes 111, 115, and 116 containing an azetidine skeleton or the 1,3-thiazinyl derivatives 98-100 are of particular interest. In this respect, it should be mentioned that as a consequence of the discovery of the cephalosporins and the enormous development occurring in the chemistry of these antibiotics since the 1940s, the 1,3-thiazine nucleus has became one of the most important six-membered heterocycles [90]. 

Dihydro-4H-l,4-thiazines from penicillin ring skeleton... 

In contrast, few reports concern dyes usable under red or near-infrared radiation (IR) laser lights in laminate. The use of the methylene blue/amine system, under the HeNe laser at 632 mn, has often been explored in the past [CAP 89]. Polymethines, thiazines, squarylium-triazine dyads have been proposed for the manufacture of HR and HOE. Some Dye/iodonium salt/silane PISs have also been mentioned but the efficiency in FRP, contrary to that attained in FRPCP, still remains relatively low under air CCRP and TEP, however, are feasible (e.g. [XIA 15] see section 1.3.12 and 1.3.13). A novel compoimd based on a dye-iodonium salt intra ion-pair (where the dye derives from a l,3-bis(dicyanomethylene) indane skeleton) works under a 635 nm light with an efficiency slightly lower than that of BM/amine [TEL 14]. 

The presumed mechanism for the formation of spirodioxo-indene-2,6 -[l,3]thiazine derivatives (156a—e,Scheme 49) can be explained as due to a nucleophilic attack of iV of 26a—e on one of the cyano groups of 3, followed by intramolecular attack of an SH group on C-2 of the indanedione skeleton in 155 (Scheme 49,12CP295) to give products 156. 

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