Synthesis of a trans-benzo-[f]-heterodecalin by catalytic hydrogenation. The important parameters for high stereo- and chemoselectivity
✍ Scribed by Martin Studer; Peter Baumeister; Hans-Ulrich Blaser; Joe Bach
- Publisher
- Elsevier Science
- Year
- 1995
- Tongue
- English
- Weight
- 970 KB
- Volume
- 95
- Category
- Article
- ISSN
- 1381-1169
No coin nor oath required. For personal study only.
✦ Synopsis
Racemic trans-1A ( translo-metboxy-1,3,4,%,5,1Ob-hexahydro-2H-[ 11 benzopyrano [ 3,4-b] pyridine, see Scheme 1) was obtained in high chemical yield (up to 89%) and with excellent stereoselectivity (95% truns) by catalytic hydrogenationdehalogenation of 7-chloro-lo-methoxy-3,4,4a,Stetrahydro-2H-[ 1 ] benzopyrano[3,4-b] pyridine 2A. The best results were obtained using W/C catalysts in dimethyl formamide (DMF) in presence of NJkliisopropylethylamine
(DIPEA) with high amounts of catalyst at low hydrogen pressure and low initial temperature. At higher initial temperatures, partial aromatization of the N-heterocycle occurred, leading on further hydrogenation to increased levels of cis-1A. Under these conditions, partial demethoxylation at the IO-position was observed as a side reaction as well. In presence of organic bases, the C = C bond was hydrogenated faster than the C-Cl bond. Inorganic bases in protic solvents favored dehalogenation over double bond reduction. Interestingly, the rate of dehalogenation decreased when the hydrogen pressure was increased. Reaction schemes are presented to rationalize these results and a comparison of our catalytic system with other methods described in the literature is made.