Expeditious synthesis of the aromatic spiroketal skeleton using hetero-Diels–Alder cycloaddition

Copper(I)‐Catalyzed Hydroalkoxylation/Hydrogen‐Bonding‐Induced Asymmetric Hetero‐Diels–Alder Cycloaddition Cascade: An Approach to Aromatic Spiroketals

“…To date, only one successful synthesis has been achieved 8p. As part of our continuing efforts directed towards the synthesis of aromatic spiroketals using a hetero‐Diels–Alder protocol,8a,b we report herein a domino process for the diastereoselective synthesis of a series of multifunctionalized spiroketal derivatives. As shown in Scheme , a metal‐catalyzed cyclization of phenols 7 provides the exocyclic enol ethers 9 , which subsequently undergo an asymmetric hetero‐Diels–Alder cycloaddition with o ‐quinone methides 10 ( o ‐QMs, generated in situ by heating their precursor o ‐methyleneacetoxyphenols 8 12) to furnish the aromatic spiroketal skeleton.…”

Synthesis of spiroacetals using functionalised titanium carbenoids

“…Exocyclic enol ethers have been used to prepare spiroacetals by cycloadditions, [3][4][5][6][7][8] or through acid-induced cyclisation of alcohols. [9][10][11][12][13] Such enol ethers have been metathesis, 22 Baeyer-Villiger oxidation of cyclic ketones 23 and by oxidation of sugars, 24 as well as by methods which would be appropriate for preparation of acyclic esters.…”

Synthesis of 5,6- and 6,6-Spirocyclic Compounds

“…An initial report [164] of the HDA between an o-QM and chroman-derived enol ethers 315 required high temperature and pressure to afford 6,6-bis(benzannulated) spiroacetals (Scheme 78). Unfortunately, the corresponding 5,6-spiroacetals could not be accessed using this method due to the difficulty in obtaining the requisite enol ether 318, which under methylenation conditions Scheme 77 Belmont et al's HDA approach to mono(benzannulated) spiroacetals [163] isomerized to the endocyclic isomer exclusively.…”

“…Scheme 78 HDA approaches toward bis(benzannulated) spiroacetals [154,164] Scheme 79 Xie et al's vinyl sulfoxide-based HDA approach toward bis(benzannulated) spiroacetals [165] Following the HDA, elimination of the sulfoxide occurs under the reaction conditions to afford the 5,6-spiroacetals 326 in good yield.…”