Inter‐ and Intramolecular Hetero Diels‐Alder Reactions, Part XXVI. Diastereoselectivity and Kinetics of Intermolecular Hetero Diels‐Alder Reactions under High Pressure. A Significant Pressure‐Induced Increase in Stereoselectivity

Abstract

The Hetero Dieb-Alder reaction of the enamino ketones Sa-d and ethyl vinyl ether (2) to give the dihydropyrans 6a-d and 7n-d is studied in dichloromethane and in hept urene under high pressure up to 7 kbar at temperatures between 0.5 and 130°C. High pressure has been applied to many transformations in order to increase their rates2). Thus all reactions which display a negative A V * , can be accelerated under high pressure. Activation volumes of about -25 cm3/mol have been found for the intramolecular and of -25 to -45 cm3/mol for intermolecular Diels-Alder reactions2''. Because of the relevance of selective organic synthesis, increasing interest has been addressed to the influence of pressure on relative rates in the formation of stereoisomers. Earlier attempts, however, to induce diastereoselectivity by high p r e s~u r e~,~) were disappointing in that the observed absolute values of AAV' did not exceed 1 cm3/ mol. We now show, that a significant and synthetic useful increase in stereoselectivity may be obtained by applying high pressure.In some recent publications we investigated the hetero DielsAlder reaction" of enamine carbaldehydes 1 and enamino ketones 5 with vinyl ethers such as 2 leading to 3-amino sugars of the garosamine and daunosamine type6.'). The cycloaddition of 1 and 2 to give the dihydropyrans 3 and 4 was studied by direct quan- Werte konnte bei der Reaktion von 5a mit 2 zu 6a/7a eine Erhiihungder Selektivitiit von 1.67: 1.00 bei 90°C und Normaldruck auf 13.6: 1.Q bei 05°C und 6 kbar erreicht werdea Dieses Beispiel migt, da13 eine signifikante und synthetisch wertvolle Erhiihuag der Diastereoselektivit chemischer Reaktionen durch Anwendung von bohem Druck m6glich ist titative infrared spectroscopy under high pressure, and an activation volume of -(25.1 f 1.7) cm3/mol could be determined in dichlorornethane*'. This newly developed method allows a detailed study of organic reactions in compressed fluid phase.In this paper we describe thc kinctics and the diastereoselectivity of the cycloadditions of 5a-d and ethyl vinyl ether (2) to give the dihydropyrans 6a-d and 7a -d under high pressure. A part of this work (reaction of 5a and 2) has already been published as a communication ' ).Chem. Ber. 122 (1989) 11 79 -11 86 0 VCH Verlagsgesellschaft mbH, D

Synthesis of 3,4-dihydro-2H-pyrans by hetero-Diels–Alder reactions of functionalized α,β-unsaturated carbonyl compounds with N-vinyl-2-oxazolidinone

“…NMR spectra: Bruker AMX 500 ( 1 H: 500.14 MHz, 13 C: 125.76 MHz) in CDCl 3 , DMSO with TMS as an internal standard. 13 C signal assignments were confirmed by XHCORR and DEPT methods. Mass spectra: Finningan Mat 95 (70 eV).…”

“…Cycloadducts 3, 5, 6 were characterized by 1 H, 13 C NMR, IR, and mass spectra. The relative cis and trans configurations of 2H-dihydropyrans cis 3a-e, trans 3d-e and trans 5a-c were assigned on the basis of 1 H NMR spectra.…”

[4+2]/HyBRedOx Approach to C‐Naphthyl Glycosides: Failure in the Projuglone Series and Reinvestigation of the HyBRedOx Sequence

“…We selected for this study six heterodienes, all of which displayed a heterosubstituent able to deliver a hydroxy ( 13a , b ) or amino ( 13c – f ) group at the C‐4' position of the target C ‐glycoside (Figure 2). These “prosugar” heterodienes were prepared according to literature procedures,17–19 except for 13d , which was obtained by treatment of 13b with dibenzylamine.…”

A New Method for the Synthesis of 3-Thiocyanatopyrazolo[1,5-a]pyrimidines

“…MeCN (99.9%, for HPLC), EtOH (95%, for analysis), water (for analysis), toluene (99+%, extra pure), EtOAc (99+%, extra pure), NH 4 SCN ( 2 ) (99+%, extra pure), KSCN (98%, pure), NaSCN (98%, extra pure), NaClO 4 (98%, extra pure), Na 2 SO 4 (99%, extra pure, anhydrous), HCl (32% solution in water, for analysis,), H 2 SO 4 (96% solution in water, for analysis,), AcOH (99.6%, for analysis), 1,1,3,3-tetraethoxypropane (97%) ( 4a ), 2,4-pentanedione (99+%) ( 4b ), 1,1,1-trifluoro-2,4-pentanedione (98%) ( 4e ), 4,4,4-trifluoro-1-phenyl-1,3-butanedione (99%) ( 4g ), 4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione (99%) ( 4h ) (Acros Organics, Geel, Belgium) were used as purchased. 3-Methyl-1 H -pyrazol-5-amine ( 1a ), 3-cyclopropyl-1 H -pyrazol-5-amine ( 1b ), 1,1,1-trichloro-4-ethoxybut-3-en-2-one ( 4c ), 4-ethoxy-1,1,1-trifluorobut-3-en-2-one ( 4d ), 1-cyclopropyl-4,4,4-trifluorobutane-1,3-dione ( 4f ) were prepared using reported [ 35 , 36 , 37 , 38 ] procedures. More spectral data can be found at Supplementary Materials section.…”