Diastereodivergent Synthesis of Trisubstituted Alkenes through Protodeboronation of Allylic Boronic Esters: Application to the Synthesis of the Californian Red Scale Beetle Pheromone

Abstract

E-allylic boronic esters undergo a highly diastereoselective protodeboronation with TBAF⋅3 H(2)O to give Z-trisubstituted alkenes. The selectivity can be switched to give predominantly the E-alkene instead by using KHF(2)/TsOH (see scheme). The utility of the methodology has been illustrated in a short synthesis of a component of the sex pheromone of the Californian red scale beetle.

trans‐Hydroboration of Propargyl Alcohol Derivatives and Related Substrates

“…Although arene rings are potentially able to intercept the ruthenium catalyst in the form of inert η 6 ‐adducts, substrates comprising substituted or unsubstituted phenyl rings worked well ( 19 – 21 ). The structure of alkenyl boronate 21 in the solid state confirms the constitution and assignment of the double bond geometry (Figure ) . It is interesting to note that there is no interaction between the MOM group and the Lewis‐acidic boron center of the B(pin) unit as reflected in a sum of angles around the boron atom of exactly 360°.…”

“…Within the limits of detection, trans ‐hydroboration of 7 a – c catalyzed by [Cp*RuCl] 4 in CH 2 Cl 2 as the preferred solvent gives the product as a single isomer, whereas the reaction of 7 d is slightly less selective. The boronate unit was invariably placed in the distal carbon atom of the reacting propargylic unit (“β‐ trans ”) . It is noteworthy that this outcome is complementary to the hydroxy‐directed trans ‐hydrostannation of the analogous propargyl alcohol 4 , which strongly favors the corresponding α‐ trans adduct 5 (Scheme ) .…”

Synthesis and Utility of Dihydropyridine Boronic Esters

“…However,w hen the unpurified dihydropyridine was treated with an acidic resin prior to hydrogenation, we obtained the tetrahydropyridines 58 and 59.Longer reaction times yielded the piperidine boronic esters 63 and 64.W ehypothesized that ab ase that is present in the reaction medium, could be promoting protodeborylation, and that Amberlyst 15 could be quenching this base. [18] Indeed, addition of LiOH in the reduction of the tetrahydropyridine 58 generated the deborylated product 56. [13] Finally,both the tetrahydropyridines and the piperidines could be oxidized with peroxide to form tertiary alcohols (60, 61 and 65, 66).…”

Highly Selective Allylborations of Aldehydes Using α,α‐Disubstituted Allylic Pinacol Boronic Esters

“…Thus, if R 3 was H, Me, or CH 2 OTBS [12] high selectivity was always observed (Table 1, entries 1-7). The reaction in entry 4 was conducted on gram scale with similar yields and identical selectivities.…”