Stereoselective Synthesis of the CDE Ring System of Antitumor Saponin Scillascilloside E-1

Unusual Transformations of Strain-Heightened Oxetanes

Oxetanes are important motifs for drug discovery and are valuable templates in organic synthesis. Much of their use as synthetic intermediates exploits their inherent strain, often resulting in chain extensions at the expense of the heterocycle. Modifications on the carbon alpha to the oxygen of oxetanes, such as the C═O of β-lactones, extend the modes of reactivity. Nevertheless, the outcomes are still largely predictable. On the other hand, other alpha modifications, such as a ═CH₂, a spiro-oxiranyl moiety, or a spiro-cyclopropyl group, increase strain and open pathways not available to simple oxetanes or β-lactones. Methods in generating 2-methyleneoxetanes, 1,5-dioxaspiro[3.2]hexanes, and 4-oxaspiro[2.3]hexanes have been developed by us and others. To date, reactions of these systems have sometimes been predictable, but often the outcomes have been unexpected. This has provided fertile ground for thinking about what controls reactivity and what other reaction pathways might be accessible to these strain-heightened oxetanes.This Account summarizes the published literature on the most straightforward approaches to 2-methyleneoxetanes, dioxaspirohexanes, and oxaspirohexanes and on their reactivity. In contrast to simple oxetanes, reactions of 2-methyleneoxetanes with nucleophiles at C4 release an enolate rather than an alkoxide. Also, 2-methyleneoxetanes can be converted to homopropargyl alcohols or undergo a silicon accelerated isomerization/electrocyclic ring opening, processes accessible only because of the exocyclic double bond. In addition, oxetane oxocarbenium ions, derived from protonation of the enol ether, can react with nucleophiles to provide 2,2-disubstituted oxetanes. Oxaspirohexanes are readily prepared by Simmons-Smith cyclopropanation of 2-methyleneoxetanes. These unusual systems undergo a variety of substituent dependent rearrangements in the presence of the Lewis acid BF₃·Et₂O. In addition, upon treatment with Zeise's dimer, oxaspirohexanes are transformed to synthetically useful 3-methylenetetrahydrofurans. Dioxaspirohexanes are easily accessed by dimethyldioxirane oxidation of 2-methyleneoxetanes. Predictably, dioxaspirohexanes react with many nucleophiles to give α-functionalized-β'-hydroxy ketones. Unexpectedly, 2,2-disubstituted oxetanes can also be selectively produced. This latter pathway has led to further unusual transformations, illuminating computational studies, and novel routes to biologically relevant molecules.

[Studies on the Syntheses of Bioactive Natural Products Having a Fused Ring System Based on Novel Skeletal Construction Methods]

Here, I describe a part of our efforts to develop synthetic strategies to construct bioactive natural products having a fused ring system. We have designed four chiral building blocks bearing contiguous quaternary stereocenters for the syntheses of bioactive C17-oxygenated steroids/triterpenoids and C9-oxygenated labdane diterpenoids. The compounds were stereoselectively synthesized from α-substituted glycolic acid (R)-3-methylcyclohex-2-enyl esters through Ireland-Claisen rearrangement to construct the stereocenters simultaneously. Synthetic utility of a β-type building block is highlighted by total syntheses of marrubiin (11 steps, 22%) and related seven labdane diterpene lactones, cyllenine C (12 steps, 29%), marrulactone (13 steps, 11%), marrulanic acid (14 steps, 10%), marrubasch F (12 steps, 14%), marrulibacetal (14 steps, 4%), marrulibacetal A (14 steps, 2%), and desertine (15 steps, 0.5%). These syntheses feature the construction of the [6.6.5]-tricyclic ring portion via a Pauson-Khand reaction, cleavage of the resultant cyclopentenone ring and an elongation of the C9 side chain by an epoxide opening reaction. The relative stereochemistry of desertine was determined to be 13R, 14S, 15S, 16R by some chemical conversions and NMR analysis. Further efforts toward total syntheses of oxygenated terpenoids using three other chiral building blocks and structure-activity relationship study of synthesized labdane diterpene lactones are currently underway in our laboratory and will be reported in due course.

Stereodivergent synthesis of vicinal quaternary-quaternary stereocenters and bioactive hyperolactones

Although great success has been achieved in asymmetric Claisen rearrangement for the synthesis of chiral γ,δ-unsaturated carbonyl compounds bearing vicinal tertiary-quaternary stereocenters, the development of asymmetric versions for stereodivergent construction of adjacent quaternary-quaternary stereocenters remains a formidable challenge because of the high steric hindrance. Here we report a catalytic enantioselective dearomatization Claisen rearrangement of allyl furyl ethers catalyzed by chiral N,N'-dioxide-Ni^II complex catalysts. A variety of chiral γ,δ-unsaturated carbonyl compounds bearing vicinal quaternary-quaternary stereocenters were obtained with excellent outcomes under mild conditions. Furthermore, we disclosed that by matching the configuration of the catalysts and the alkene unit of the substrates, four stereoisomers of the products could be prepared in excellent yields and stereoselectivities. Finally, the fascination of this strategy was demonstrated by stereodivergent synthesis of bioactive natural products hyperolactones B, C, and their epimers. A possible catalytic model was proposed to explain the origin of the asymmetric induction.