Activated cycloheptenone dienophiles. A versatile approach to 6,7-fused ring targets

Abstract

Several 2-carbalkoxy-2-cyclohepten-I-ones were prepared and their Diels-Alder characteristics examined in order to develop a facile general approach to the synthesis of cycloheptane ring-containing polycyclic natural products. Substitution pattern on the seven-membered ring was found to greatly affect the Diels-Alder behavior. In addition, some extraordinary facial stereoselectivity was observed.

Selective Carbon–Carbon Bond Cleavage of Cyclopropanols

“…The endocyclic ring opening is consistent with fragmentation to generate the more stable anion at the α-position of the ester. The authors considered a number of alternative methods for one-carbon ring expansion in this synthesis, including treatment with stabilized carbenoids, , nonstabilized carbenoids, ethyl diazolithioacetate, and silyloxycyclopropane homologation. , Other cyclopropyl alcohol ring-opening reactions were also unsuccessful, such as treatment of a cyclopropyl alcohol with 1,8-diazabicyclo[5.4.0]­undec-7-ene (DBU). Thus, the use of the Zercher-type ring-expansion protocol of homoenolates facilitated formation of the desired product from the authors’ late-stage intermediate ( 55 ).…”

Towards the Total Synthesis of Vibsanin E, 15‐O‐Methylcyclovibsanin B,3‐Hydroxyvibsanin E, Furanovibsanin A, and 3‐O‐Methylfuranovibsanin A

“…Attempts with stabilized carbenoids (e.g. EtO 2 CCHN 2 ), known to insert regioselectively at the least-substituted carbon alpha to ketones, using BF 3 ·Et 2 O [26] or Meerwein's salt [27] failed, as did non-stabilized carbenoids, [28] ethyl diazolithioacetate [29] and silyloxycyclopropane homologation. [30] Considering BeckwithDowd [31] and variant [32] ring expansion protocols are widely reported, 26 was converted, using Mander's reagent [33] and subsequent retro Dieckmann/Dieckmann reaction, to 36 (X-ray crystal structure of α-CO 2 Et, see Figure 4) in 69 % overall yield.…”

Total Synthesis of (−)‐Rotundone and (−)‐epi‐Rotundone from Monoterpene Precursors

“…For the assignment of the diastereomers of product 8 , the cis ‐ and trans ‐isomers of commercially available rac ‐dihydrocarvone 7 were separated by flash chromatography [24] . BF 3 (OEt) 2 ‐promoted diazo acetate insertion into the separated isomers 7a and 7b occurred preferentially with migration of the less substituted carbon−carbon bond in α‐position to the ketone, as known from similar substrates under these conditions [25–27] . Whereas diazoacetate insertion into trans ‐dihydrocarvone 7a gave only one regioisomer, 8a , cis ‐dihydrocarvone substrate 7b generated also regioisomer 8c as byproduct in an (at this stage) inseparable mixture of 8b and 8c .…”