Spiroacetals of Marine Origin

Divergent access to polycyclic spiro- and fused-N,O-ketals through Bi(OTf)3-catalyzed [4+2]-annulation of cyclic N-sulfonyl ketimines and alkynols

Bismuth(III) triflate-catalyzed [4+2]-annulation of cyclic N-sulfonyl ketimines (derived from saccharin) and alkynyl alcohols (4-pentyn-1-ols and 5-hexyn-1-ols) has been reported. This cascade annulation provides a diverse array of polycyclic spiro-and-fused N,O-ketals with excellent substrate scope, good isolated yields, and diastereoselectivities under mild reaction conditions.

Regioselective Synthesis of Benzannulated [5,6]-Oxaspirolactones via Cu(II)-Catalyzed Cycloisomerization of 2-(5-Hydroxyalkynyl)benzoates

Spiroketals and oxaspirolactones are widely found in biologically active natural products, serving as important structural motifs. In this study, we present a Cu(II)-catalyzed cascade cycloisomerization of 2-(5-hydroxyalkynyl)benzoates, enabling the regioselective synthesis of benzannulated [5,6]-oxaspirolactones containing an isochromen-1-one moiety. This strategy offers a rapid and efficient approach to access a diverse array of benzannulated [5,6]-oxaspirolactones. The methodology presented here showcases a broad substrate scope, delivering good yields and scalability up to gram scale. The structures of the oxaspirolactones were unequivocally confirmed through single-crystal X-ray analysis and by analogy using 1H and 13C{1H} NMR data.

Ready Access to Benzannulated [5,5]-Oxaspirolactones Using Au(III)-Catalyzed Cascade Cyclizations

This work showcases an unprecedented Au(III)-catalyzed cascade cyclization of 2-(4-hydroxyalkynyl)benzoates to access benzannulated [5,5]-oxaspirolactones related to biologically active natural products. This reaction proceeds through an initial 5-endo-dig mode of hydroalkoxylation of the alkynol segment to give the oxocarbenium species (via cyclic enol-ether) followed by the addition of carboxylate onto the oxocarbenium that delivers the oxaspirolactone scaffold. While testing this method's scope, we found that the steric and electronic environment of the hydroxyl group could alter the reaction pathway that delivers isochromenone through a competitive 6-endo-dig mode of attack of the carboxylate onto the tethered alkyne.

Re2O7-Catalyzed Approach to Spirocyclic Ether Formation from Acyclic Precursors: Observation of Remote Stereoinduction

Ketones that are flanked by an allylic alcohol and an alkene isomerize to spirocyclic ethers in the presence of Re2O7 through allylic alcohol transposition, oxocarbenium ion formation, and Prins cyclization. These processes provide significant increases in molecular complexity, with multiple stereocenters being set relative to a stereocenter in the substrate. Stereoselectivity arises from the initial reversible steps being more rapid than the final step, thereby allowing for thermodynamically controlled stereochemical equilibration prior to product formation.

Bicyclic acetals: biological relevance, scaffold analysis, and applications in diversity-oriented synthesis

The chemoinformatics analysis of fused, spiro, and bridged bicyclic acetals is instrumental for the DOS of natural product-inspired molecular collections.

A bidirectional synthesis of spiroacetals via Rh(ii)-catalysed C-H insertion

Acyclic methylene acetals bearing two diazoester subunits have been converted to [5,5]-spiroacetals via bidirectional C-H insertion under Rh(ii) catalysis. Using a chiral Rh(ii) catalyst, the major diastereomer can be produced in high enantiomeric excess (89%).

Lewis acid catalyzed cascade annulation of alkynols with α-ketoesters: a facile access to γ-spiroketal-γ-lactones

A simple protocol for the synthesis of diverse unsaturated γ-spiroketal-γ-lactones has been developed by employing a Bi(OTf)₃ catalyzed cascade annulation of alkynols with α-ketoesters via a dual (π and σ) activation process.

Spiroacetal formation through telescoped cycloaddition and carbon-hydrogen bond functionalization: total synthesis of bistramide A

Spiroacetals can be formed through a one-pot sequence of a hetero-Diels-Alder reaction, an oxidative carbon-hydrogen bond cleavage, and an acid treatment. This convergent approach expedites access to a complex molecular subunit which is present in numerous biologically active structures. The utility of the protocol is demonstrated through its application to a brief synthesis of the actin-binding cytotoxin bistramide A.

Diacetoxyiodobenzene-mediated synthesis of unnatural furospirostane sapogenins derived from diosgenin and tigogenin

Two unnatural steroid sapogenins bearing a furospirostane side chain were prepared starting from the readily available spirostane sapogenins, tigogenin and diosgenin following a synthetic protocol that included: (i) introduction of a carbonyl group at position C-23, (ii) diacetoxyiodobenzene-induced F-ring contraction and (iii) LiAlH4 reduction of the newly emerged methoxycarbonyl moiety. The structures of the new compounds were corroborated by NMR and X-ray studies.

Mechanistic insights on the reactivity of furospirostanes with the 16β,22:22,25-diepoxy-23-acetoxymethyl-24-methyl side chain

F-ring opening in spirostanes with the 16β,22:22,25-diepoxy-23-acetoxymethyl-24-methyl side chain produces a Δ(22)-intermediate with an allylic acetoxy group. For this reason the reactivity profile of these compounds deviates from that observed in other naturally occurring or synthetic spirostanes and furospirostanes.