Stereoselektive Protonierung von Carbanionen, 3. 1,3‐Dioxolan‐4‐one und 1,3‐Oxazolidin‐4‐one: Synthesen und diastereoselektive Protonierung ihrer Anionen

Abstract

Dioxolonanones 3 and oxazolidinones 4 are synthesized by -78°C occurs by kinetic product control yielding always ca. different methods together with the silyl enol ether 3aa. The 70-90% of the cis isomers. Exchange of the cation in 3Ll by configuration of their diastereomers is determined by 'H-potassium or addition of several Lewis acids does not effect NMR spectroscopy and relative retention times (vpc), backed the cidtrans relations. Obviously, even the steric effect of a by a crystal structure analysis of cis-3d. Protonation of 3a,, methyl group in 2-position of 3 , , and 4,, dominates so stronand 4aL1 by different OH and CH proton sources in THF at gly that the effect of other parameters are suppressed.In der 1. Mitteilung dieser Reihe berichteten wir iiber systematische Versuche zur diastereoselektiven Protonierung der exocyclisch stabilisierten Carbanionen, die sich von den Cyclohexanderivaten 1 ableiteni3]. Als Gegenstiick dazu wurden die endocyclisch stabilisierten Carbanionen der Bis-lactimether 2 untersucht['l.Nunmehr erweitern wir diese Studie auf die endocyclischen Enolat-Ionen aus den 1,3-Dioxolan-4-onen 3 und den 1,3-Oxazolidinonen 4, die bisher vonviegend unbekannt waren. Es sollte vor allem gepriift werden, ob in diesem starren Fiinfring-Enolaten bereits R = Me fur eine starke Seitendifferenzierung bei der Protonierung ausreicht, nachdem sich die Anionen von 3 [R = 3dr61 und andere Dioxolane mit R = C(CH3), [61] und verwandte Derivate von 4, namlich N-Acyl-2-tert-butyloxazolin-4-one~7~ hoch stereoselektiv alkylieren lassen. SchlieDlich ist 3a eng mit dem 2,2-Dimethylderivat (2-Me statt 2-H in 3a) verwandt, das ein vorziigliches Modellsystem zur enantioselektiven Protonierung von Enolat-Ionen darstellt [8].

1,3-Dipolar cycloaddition of azomethine ylides derived from imines and difluorocarbene to alkynes: a new active Pb-mediated approach to 2-fluoropyrrole derivatives

“…The structure of the products was assigned on the basis of spectral data and comparison of their 1 H NMR spectra with those reported in the literature. 17, 18 The reaction of N-benzylideneaniline with difluorocarbene in the presence of two dipolarophiles, DMAD and phenylpropynal, taken in an equimolar ratio, gave a mixture of fluoropyrrole 4a and both stereoisomeric oxazolidinones 24 and 25 (4a : (24 ϩ 25) 5.5 : 1). This suggests the C᎐ ᎐ ᎐ C bond of DMAD to be substantially more active than the aldehydic carbonyl in reactions with the ylide 2a.…”

Concave Reagents. 20. Sterically Shielded m-Terphenyls as Selective Agents in General Protonations1

“…If a carbanion is part of a ring system that carries appropriate substituents protonation can lead to cis - and trans -isomers . The cis / trans -ratio should be controllable if a general protonation was possible.…”

“…We have tried to extend this selective protonation to ester stabilized cyclic carbanions for which diastereoselective protonations have already been investigated by Hünig et al4b (see Table ). In the case of the methyl ester they found remarkable “uniform” cis / trans -selectivities (max 73:27).…”

“…Selective protonations have been investigated thoroughly. Some progress has been made toward enantioselectivity but even the problem of diastereoselective or regioselective protonation is not solved yet . To increase the selectivities of protonations, concave acids and other sterically extremely shielded acids have been developed .…”