Asymmetrische Homoaldol-Reaktion durch enantioselektive Lithiierung eines prochiralen 2-Butenylcarbamats

Unusual carbamate-directed CH-activation at an annulated ferrocenophane framework

The tetrahydroazepine-annulated [3]ferrocenophane carbamate (4) was synthesized by two different linear routes starting from the readily available α-dimethylamino[3]ferrocenophane-ortho-carbaldehyde rac-6. The carbamate directed lithiation of 4 resulted in a selective attack at a (Cp)C-H bond at the higher substituted "lower" [3]ferrocenophane Cp-ring to eventually yield the respective ester (18) after treatment with ClCO(2)Me.

Total synthesis of discodermolide: optimization of the effective synthetic route

An efficient and modulable total synthesis of discodermolide (DDM), a unique marine anticancer polyketide is described including related alternative synthetic approaches. Particularly notable is the repeated application of a crotyltitanation reaction to yield homoallylic (Z)-O-ene-carbamate alcohols with excellent selectivity. Advantage was taken of this reaction not only for the stereocontrolled building of the syn-anti methyl-hydroxy-methyl triads of DDM, but also for the direct construction of the terminal (Z)-diene. Of particular interest is also the installation of the C13=C14 (Z)-double bond through a highly selective dyotropic rearrangement. The preparation of the middle C8-C14 fragment in two sequential stages and its coupling to the C1-C7 moiety was a real challenge and required careful optimization. Several synthetic routes were explored to allow high and reliable yields. Due to the flexibility and robust character of this approach, it might enable a systematic structural variation of DDM and, therefore, the elaboration and exploration of novel discodermolide structural analogues.

Development of practical syntheses of the marine anticancer agents discodermolide and dictyostatin

Initially isolated in trace quantities from deep-sea sponges, the structurally related polyketides discodermolide and dictyostatin share the same microtubule-stabilizing antimitotic mechanism as Taxol. Discodermolide has been the focus of intense research activity in order to develop a practical supply route, and these efforts ultimately allowed its large-scale synthesis and the initiation of clinical trials as a novel anticancer drug. Similarly, the re-isolation and synthesis of dictyostatin continues to stimulate the biological and chemical communities in their quest for the development of new chemotherapeutic agents. This comprehensive review chronicles the synthetic endeavours undertaken over the last 15 years towards the development and realization of practical chemical syntheses of discodermolide and, more recently, dictyostatin, focusing on the methods and strategies employed for achieving overall stereocontrol and key fragment unions, as well as the design and synthesis of novel hybrid structures.

Highly Enantioenriched Homoenolate Reagents by Asymmetric γ-Deprotonation of Achiral 1-Silyl-Substituted 1-Alkenyl Carbamates

1-trimethylsilyl-1-alkenyl carbamates 1 are deprotonated by n-butyllithium/(-)-sparteine (2) with a high degree of enantiotopic differentiation in the gamma-position to form the enantiomerically enriched allyllithium derivatives 3. Trapping these with several electrophiles proceeds stereospecifically in an anti-S(E)' or syn-S(E)' substitution to form products 4 or ent-4, respectively. Compounds 3a (R = Me) and 3b (R = Ph) exhibit toward carbonyl electrophiles opposite senses of almost complete stereospecificity, thus for 3b.2 the involvement of a eta(3)-complex is suggested. [reaction: see text]

Synthesis of Enantiomerically Enriched Allenes by (−)-Sparteine-Mediated Lithiation of Alkynyl Carbamates

[reaction: see text]. The alpha-deprotonation of alkynyl carbamates 3 with the chiral base (-)-sparteine (4)/n-butyllithium, transmetalation with ClTi(O(i)()Pr)3, and subsequent substitution with an aldehyde results in the formation of enantioenriched 4-hydroxyallenyl carbamates 11. Stereoselection is determined by dynamic resolution of the lithium/(-)-sparteine complexes by selective crystallization.