Tetrahydro analogue

Hydrogenation of 3-methylpyrido[2,3- ]pyrazin-2(37/)-one 211 by the action of NaBH4 in NaOH gave 212 <1995JHC703>. On the other hand, the tetrahydro analogue 214 was obtained from 213 by the action of H2/ Raney-Ni in THF <1996JHC1737>. 

Isoxazoles and their dihydro and tetrahydro analogues serve as immensely flexible building blocks in synthesis through their ability to function as masked forms of /3-diketones, /3-hydroxyketones (and thus enones) and y-amino alcohols. All of these interrelationships are possible because of the relatively labile nature of the nitrogen-oxygen bond. 

Sometimes the reaction is followed by oxidation and leads to 1,2-dihydro-pyridines instead of their 1,2,3,4-tetrahydro analogues. As often as not, such a situation is observed in the case of cyclic unsaturated ketones in the presence of triethylamine or piperidine [137,138,139, 140, 141, 142, 144, 147,148,149,150, 151] (Scheme 3.41). The possibility of such reactions occurring instead of a,(3-... 

The 3-methylene unit in both the 2,2-dimethyldihydrothiopyran-4-one 141 and the tetrahydro analogue is poorly resolved such that the alkene signals appear as broadened singlets. The increased planarity of the thiopyran ring in 142 is manifest in a slight downfield shift of H-2 to 53.87 compared with 53.56 in the tetrahydro compound <1996SL261>. 

Examples of either the dihydro or tetrahydro analogues of the heterocycles in this chapter are rare. A synthesis of 3-aryl-4-methyl-4,5,6,7-tetrahydro-3/7-l,2,3-triazolo[4,5-6]pyridines was reported (see Section 7.10.9.1.1), but its chemistry was not discussed <86CB359i>. 

Another approach to the synthesis of condensed azine derivatives (84UK1648 85KGS1011) is based on the ability of a number of azine derivatives to undergo diaddition reactions with nucleophiles. When bifunctional nucleophilic reagents are used, these additions result in the formation of cyclization products. In all these reactions the tetrahydro analogues of the fully unsaturated heterocyclic systems are obtained. 

Similarly, heterocyclic systems 47-49, the tetrahydro analogues of those obtained by reactions of 2,3-dichloroquinoxaline with /3-dicarbonyl compounds, thioamides, or dithiocarbamates, have also been prepared (Scheme 42) (840MR775 85KGS1011). 

The formulation of compounds 169 and 171 as 3H- and not IH-derivatives is supported by their PMR spectra. In CDCI3, compound 171 shows a one-proton singlet at 8 6.59 which is not deuterium exchangeable in sodium deuteroxide-deuterium oxide. This signal is assigned to the benzylic proton in the 3-position. The tetrahydro analogue 169 (Ar = Ph) shows a similar signal at 5 5.54. The ultraviolet spectra of compounds 169 and 171 have also been discussed. In methanol the dihydro derivative 171... 

Benzodiazepines can be reduced to the dihydro or tetrahydro analogues either by catalytic hydrogenation or with sodium borohydride. Both dihydro- and tetrahydro-l,5-diazepin-2-ones are reduced to the tetrahydrodiazepines with lithium aluminum hydride . 

Scheme 29 Preparation of 1,2- and 1,4-dihydrophosphinine oxides 142 and 143, and 1,2,3,6-tetrahydro analogues 144 and 145...

Thlazolo[3.2-a][1.31diazepines T C NS-C N 1 Cyclocondensation of hexahydro-2H-l,3-diazepin-2-thione with a-bromoketones gives 2,3-disubstituted 2,3,5,6,7,8-hexahydrothiazolo[3,2-a]-1,3-diazepin-2-ols. A similar method is used for tetrahydro analogues . ... 

Amino-2,1,3-benzothiadiazole (237) is convertible into its anthranilic acid derivative (238), which is cyclized by phosphorus oxychloride to 6-chloro-[l,2,5]thiadiazolo[3,4-c]acridine (239). Replacement of its 6-halogeno-substituent produces derivatives such as (240). An analogous series of reactions provides 7,8,9,10-tetrahydro-analogues. The condensation of (237) and alkyl 2-oxocyclopentanecarboxylate yields derivatives of the [1,2,5]-thiadiazolo[3,4-h]quinoline ring system such as (241)—(243). Electrophilic substitutions of naphtho[l,2-d][2,l,3]thiadiazole (244) have been studied in some detail. Nitration produces a mixture of the 6- and... 

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