No abstract

A. Erfahrungen bei Synthesen der a-Alkyl-y-lactone Wege zu einfachen 2-Alkyl-4-butanoliden sind vorwiegend in der dteren Literatur publiziert und deswegen haufig nicht mit optimalen Synthesevorschriften beschrieben 6). Das kaufliche, jedoch teure a-Methyl-y-butyrolacton (1 b) erhielten wir in 80proz. Ausbeute nach abgewandeltem Vorbild',*) durch Deprotonierung des Grundkorpers 1 a mit nur 1.05 Aquiv. LDA in THF (15 min bei -65"C), anschliel3ende Zugabe von 1.26 Aquiv. Methyliodid und Isolierung uber die Natronlauge-Phase'). Fur groBere Ansatze im mol-MaBstab eignet sich dieses Verfahren nicht sehr, weil das dann auch bei sor~altiger Reaktionsfiihr~ng~~~) gebildete up-Dimethyl-y-butyrolacton und das nicht methylierte l a durch Destillation nur schwer abgetrennt werden konnen. In Umkehrung der publizierten") Reaktionsfolge (64%) bereiteten wir 1 b rationeller in 8lproz. Reinausbeute") aus 1.51 mol des Natrium-Salzes von Methylmalonsaure-diethylester und 1.63 mol Ethylenoxid uber 2a mit Zwischenisolierung der beschriebenen") Lactondure 2 b. Isobutenoxid lieferte im gleichen Verfahren das fruher12-'" anders hergestellte 2,4-Dimethyl-4-pentanolid (62% lc), das sich (ebenso wie 1 d) im Unterschied zu 1 b in verd. Natronlauge erst beim Erhitzen unter Ringoffnung lost. Das reaktionstragei3) trans-2-Butenoxid reagierte nach diesem Verfahren nicht mehr, wohl aber noch mit dem Natnum-Salz von Malonsaure-dimethylester " 1.

Short-lived pivaloylmetals, (H(3)C)(3)C-COM, were established as the reactive intermediates arising through thermal heterolytic expulsion of O=CtBu(2) from the overcrowded metal alkoxides tBuC(=O)-C(-OM)tBu(2) (M = MgX, Li, K). In all three cases, this fission step is counteracted by a faster return process, as shown through the trapping of tBu-COM by O=C(tBu)-C(CD(3))(3) with formation of the deuterated starting alkoxides. If generated in the absence of trapping agents, all three tBu-COM species "dimerize" to give the enediolates MO-C(tBu)=C(tBu)-OM along with O=CtBu(2) (2 equiv). A common-component rate depression by surplus O=CtBu(2) proves the existence of some free tBu-COM (separated from O=CtBu(2)); but companion intermediates with the traits of an undissociated complex such as tBu-COM & O=CtBu(2) had to be postulated. The slow fission step generating tBu-COMgX in THF levels the overall rates of dimerization, ketone addition, and deuterium incorporation. Formed by much faster fission steps, both tBu-COLi and tBu-COK add very rapidly to ketones and dimerize somewhat slower (but still fairly fast, as shown through trapping of the emerging O=CtBu(2) by H(3)CLi or PhCH(2)K, respectively). At first sight surprisingly, the rapid fission, return, and dimerization steps combine to very slow overall decay rates of the precursor Li and K alkoxides in the absence of trapping agents: A detailed study revealed that the fast fission step, generating tBu-COLi in THF, is followed by a kinetic partitioning that is heavily biased toward return and against the product-forming dimerization. Both tBu-COLi and tBu-COK form tBu-CH=O with HN(SiMe(3))(3), but only tBu-COK is basic enough for being protonated by the precursor acyloin tBuC(=O)-C(-OH)tBu(2) .

Dilithiumcarbonsaure-Dianionen 1 addieren sich an a,P-ungesattigte Aldehyde (2) 1,2-regiospezifisch unter Bildung der y,&ungesattigten PHydroxycarbonsauren 3/4, wobei im Falle des Phenylessigsaure-Dianions ( l a ) die Addition durch reversible Reaktionsfuhrung zu hoher threoSelektivitat gesteuert werden kann. -a.PEnone (8) nehmen 1 irreversibel in 1,2-und 1,CPosition auf, wobei nach Mal3gabe der Substituenten das gesamte Spektrum von reinem 1,2-zu reinern 1,4-Addukt iiberstrichen wird. Bei gleichbleibendem Substitutionsrnuster steigt der 1,4-Anteil mit der komplexierenden Wirkung der Gegenionen von 1 und mit der Lewis-Basizitat des Solvens. Additions of Carboxylic Acid Dianions to a,P-Unsaturated Carbonyl Compounds - Control of the 1,2-/1,4-Regioselectivity by Steric Substituent EffectsDilithium carboxylic acid .dianions 1 attack a,(3-unsaturated aldehydes (2) 1,2-regiospecificaIly with formation of the unsaturated (3-hydroxy carboxylic acids 314. Additionally, the addition of the phenylacetate dianion l a can be conducted to the threo-isomer with high selectivity by reversible reaction. -a,kEnones (8) add 1 irreversibly in 1,2-and 1,Cposition. Depending on the substitution pattern, the whole range from pure 1.2-to pure 1,4-adduct is covered. The influence of the 1-counterion M and of the solvent is significant; the 1,4-portion increases with the cornplexing effect of M and with the Lewis-basicity of the solvent.Die Addition resonanzstabilisierter Organometallverbindungen an a, Pungesattigte Carbonylverbindungen wurde in den letzten Jahren ungewohnlich intensiv untersucht 1). Im Mittelpunkt dieses lnteresses stand imrner wieder die Frage nach der Regiochemie (1.2-vs. 1.4-Addition) und deren Beeinflussung durch Substituenten, Gegenionen und Solvens. Wir berichten hier iiber die Urnsetzung von Carbonsaure-Dianionen 1 mit a,(3-ungesattigten Aldehyden und Enonen; der besondere Reiz dieses Systems liegt darin, dal3 durch geeignete Wahl der Substituenten am Dianion und an der Carbonylkomponente der Gesarntbereich zwischen den Extremen 1,2-bzw. 1,4-Angriff iiberstrichen wird2).

Pivaloin (2), prepared from ethyl pivalate (1) or from dimethyl oxalate (3), reacts with tert-butyllithium by reduction (45% of 10) and addition (45 % of 9), whereas the corresponding Barbier variant takes a different course. Productive transformations of the a,a,P-tri-tert-butyl glycol 9 lead to a,p,p-tritert-butylethanone (17, overall 4 steps), or a,a,P-tri-tert-butyl-ethene (20, 3 or 4 steps), or a,p,P-tri-tert-butyl-P-hydroxyethanone (19, 3 steps, also by Grignard addition to bipivaloyl 4). Steric congestion is assessed by searches for restricted internal rotation.. The alkene 20 and its epoxide 27 are studied with respect to NMR assignments and chemical degradation.We describe short routes to several a,a,P-tri-tert-butyl compounds which are either new or have been previously prepared only by long or inefficient procedures. Some of their properties are reported to show how intramolecular mobility and chemical reactivity may depend on the overcrowding caused by three tert-butyl ("tBu") groups. A. Preparation and teut-Butylation of PivaloinPivaloin (2) is the common source of all material described in this work but is commercially no longer avail-abler2]. For its preparation from ethyl pivalate (1) and sodium metal, the leading references"] are rather laconic and do not mention the considerable amount (ca. 2.50/) of yellow bipivaloyl (4) generated in the process. We preferred to run this acyloin condensation on large scales in to provide for an easier separation of 4r41 and purification of 2 that yielded at least 52% of colourless pivaloin, with the 'H NMR doublet splitting typical"] of the very clean material. Diketone 4 was certainly formed by a protolytic disproportionation of its radical anion that would have either been present as one of the primary products or perhaps resulted from rapid oxidation[6] of the expectedr71 enediolate 5. Trapping of 5 with chlorotrimethylsilane did not appear yield. 4 Na i 25% o be helpful in view of a much lower reported[7]> 52%