Tetrahydrooxazine ring

In reactions with azides, ketones are directly converted to 5-hydroxytriazolines. Ketone enolate 247, generated by treatment of norbornanone 246 with LDA at 0°C, adds readily to azides to provide hydroxytriazolines 248 in 67-93% yield. Interestingly, l-azido-3-iodopropane subjected to the reaction with enolate 247 gives tetracyclic triazoline derivative 251 in 94% yield. The reaction starts from an electrophilic attack of the azide on the ketone a-carbon atom. The following nucleophilic attack on the carbonyl group in intermediate 249 results in triazoline 250. The process is completed by nucleophilic substitution of the iodine atom to form the tetrahydrooxazine ring of product 251 (Scheme 35) <2004JOC1720>. 

Tautomerism caused the different reactivity of 3-iminotetrahydrooxazines 111 and 112 shown in Scheme 1, Section 8.06.4.2 <2001CHE1054>. Otherwise there are no examples of a tetrahydrooxazine ring having a significant effect on... 

Cononitarines A and B (505,506) are characterized by the presence of a phenolic substituent at C(ll ) instead of OMe. In 506, the side chain at the iboga C(20 ) is an ethyl group in place of a hydroxyethyl in 505. Two additional bisindoles of the same type, conodirinines A and B (507,508), in which the vobasinyl unit has incorporated an additional tetrahydrooxazine ring, were also obtained from the same plant 306). Notable differences in the NMR spectral data of 507 and 508 compared to the other... 

Much interest lies in the use of dihydro-1,3-oxazines (190) as enolate equivalents, since, if an alkyl group is carried at C-2, these compounds may be deprotonated and the anions formed reacted with numerous types of electrophiles. Reduction of the imine bond of the products (191), is then conveniently effected by treatment with sodium borohydride. The tetrahydrooxazines (192) which are formed may then be ring opened by hydrolysis with aqueous acid (Scheme 15). This topic and its utility in synthesis has been well reviewed. - ... 

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