A study of chiral induction in some diene additions catalyzed by chiral alkoxyaluminum dichlorides

Abstract

A reliable procedure for the preparation of the chiral Lewis acid catalysts, (−)-menthoxy- ((−)-1) and (−)-bornyloxy-aluminum dichloride ((−)-2), has been established for the study of diene additions. The geometry of conformationally mobile α,β-unsaturated carbonyl compounds complexed with these catalysts has been probed using the α-methylene ketones 3 and 4 and 2,6-dimethyl-1,4-benzoquinone 5 as dienophiles of fixed geometry. The addition of the first two to cyclopentadiene and (−)-1 yields exo and endo adducts that exhibit a range of 28–54% enantiomeric excess (ee); with 5, only the endo isomer, showing 8% ee, is formed. In the corresponding additions catalyzed by (−)-2, adducts displaying ee in the range of 3–18% are formed.

Chiral Aluminum Lewis Acids in Organic Synthesis

“…The only catalyst that gave higher induction than catalyst 4 derived from menthol was catalyst 144 derived from neomethyl menthol, although the magnitude of the increase (61 % ee compared with 57 % ee) is probably within experimental error. 17. Northcott and Valenta investigated the reaction of the exo-cyclic methylenedienophiles 159 and 161 and the benzoquinone 163 with cyclopentadiene [39]. The absolute configurations of the products were not determined for these three reactions but it was determined that catalyst 4 and catalyst ent-143 gave the same major enantiomer of 164 but different major enantiomers of 160.…”

“…An alternative model proposed by Northcott and Valenta involves an s-cis conformation of the dienophile as indicated in structure 171 [39]. Shielding of the re face by the iso-propyl group requires a change in the coordination of aluminum to the dienophile oxygen such that the aluminum is coordinated by the oxygen on the same side as the carbon substituent of the aldehyde.…”

Monoterpenoids

“…Lewis acid treatment of menthone oxime (131) yields the lactam (132).147 (-)-Carvone ( 133) is reduced to (+)-dihydrocarvone, enantiomer of (1 17), by sodium hydrogen telluride in ethanolic ~01ution. '~~ Either or both of the olefinic bonds in carvone may be reduced using Zn-NiCl,, depending upon the reaction conditions emp10yed.l~~ Addition of thiophenol to (-)-carvone (133) yields the Michael adduct (134) which can be converted into purely trans ( + )-dihydrocarvone, enantiomer of (1 17), via reduction to the thioalcohols (135) followed by desulphurization and then oxidation using PDC-DMF-0 OC.150 When (+)-pulegone, enantiomer of (91), is reacted with the TiC1,-Mg-THF complex the product can be (a) a 1 : 1 mixture of (-)-menthone, enantiomer of (92), and (+)isomenthone (93) if Bu'OH is present, (b) the triene (1 36) from reaction with, for example, crotyl chloride (which is not itself reduced), or (c) the dimers (137) and (138) which are formed when no other electrophile is i n t r 0 d ~c e d . I ~~ The acetoxy p-menthadienone (1 39), available via oxidation of carvacrol (89) using Pb(OAc),, could be dimerized Diels-Alder-wise in refluxing ethanolic sulphuric acid (!)…”

“…180 A reliable route to the chiral catalyst (1 53) has been described. 137 The amino-alcohol (1 54) catalyses the reaction between, for example, benzaldehyde and diethylzinc, which yields (9-1-phenylpropanol of 98-99 % e.e.18' An improved route from (+)-camphor (143) to the benzyl ether (155) has been published.lE2 Oxidation of the ally1 sulphide (1 56) using 3-chloroperoxybenzoic acid yields only the sulphoxide (1 57).IE3 When the oxidant is NaIO,, a 66 : 34 mixture of the two possible isomeric sulphoxides is obtained. The chiral diene (1 58) has been synthesized from (+ )-camphor ( 143).la4 The structure of the major product obtained by reaction of camphor (143) with hydrazoic acid has been reas~igned'~~ as (159).…”

Darstellung, Eigenschaften und Molekülstrukturen von Alkylaluminiumaminoalkoxidchloriden

“…Als Hauptprodukt der Umsetzung ist das exo-Produkt mit bis zu 72% erhalten worden [4]. Weitere Untersuchungen haben gezeigt, daû die Katalysatorwirkung chiraler Alkoxyaluminiumdichloride sehr stark von ihrer Reinheit und dem angewendeten Herstellungsverfahren beeinfluût wird [5,6]. Die Effektivita È t der chiralen Induktion bei der oben besprochenen Reaktion ist abha È ngig von der Komplexbildungstendenz zwischen den Katalysatoren und den als Ausgangsstoff eingesetzten ungesa È ttigten Aldehyden [7].…”