“…Allylsilanes are protodesilylated by Brønsted acid as either a planned procedure 17 or as an undesired side reaction . It is possible that Me 2 AlCl is serving as a proton scavenger in the present study and is essentially cleaning the TiCl 4 in situ as has been suggested previously. , Although this appears to be a reasonable explanation for the substitution results, it is unlikely that the trace amount of acid present in the reaction mixtures in this study (presumably from TiCl 4 ) could be responsible for the amount of protodesilylation observed in the Diels−Alder reaction (Table , entry 5). However, several equivalents of HCl would be produced upon aqueous catalyst quench with saturated NaHCO 3 .…”
Diels−Alder cycloaddition of
2-[(trialkylsilyl)methyl]-1,3-butadienes with a variety of
dienophiles
and substitution reactions between these allylsilane-containing adducts
and aldehyde or acid
chloride electrophiles have been combined into “tandem sequential
reactions.” These tandem
sequences proceed with equal or greater yield (50−80%) than the
reactions performed separately
with no decrease in regio- or stereoselectivity. The sequence
produces cyclic compounds with three
or four stereogenic centers with good to excellent diastereoselectivity
from three simple, noncyclic,
and achiral reaction partners. Unprecedented levels of
diastereoselectivity (de >93%) have been
achieved in allylic substitution reactions involving substituted
[(trialkylsilyl)methyl]cyclohexene
derivatives with aldehyde electrophiles. During the course of
these studies, protodesilylation of
allylsilanes has been investigated in detail, and a cocatalyst system
of TiCl4 and Me2AlCl has been
developed that has eliminated silicon loss in the substitution
reactions studied. Lewis acid-promoted
enolization of ester and ketone substrates with chiral centers adjacent
to the carbonyl moiety has
been studied also. It has been shown that this event in our
studies occurs primarily during catalyst
quench. This isomerization is prevented by quenching the catalyst
with a Lewis base, such as
methanol or triethylamine, prior to aqueous workup.
“…Allylsilanes are protodesilylated by Brønsted acid as either a planned procedure 17 or as an undesired side reaction . It is possible that Me 2 AlCl is serving as a proton scavenger in the present study and is essentially cleaning the TiCl 4 in situ as has been suggested previously. , Although this appears to be a reasonable explanation for the substitution results, it is unlikely that the trace amount of acid present in the reaction mixtures in this study (presumably from TiCl 4 ) could be responsible for the amount of protodesilylation observed in the Diels−Alder reaction (Table , entry 5). However, several equivalents of HCl would be produced upon aqueous catalyst quench with saturated NaHCO 3 .…”
Diels−Alder cycloaddition of
2-[(trialkylsilyl)methyl]-1,3-butadienes with a variety of
dienophiles
and substitution reactions between these allylsilane-containing adducts
and aldehyde or acid
chloride electrophiles have been combined into “tandem sequential
reactions.” These tandem
sequences proceed with equal or greater yield (50−80%) than the
reactions performed separately
with no decrease in regio- or stereoselectivity. The sequence
produces cyclic compounds with three
or four stereogenic centers with good to excellent diastereoselectivity
from three simple, noncyclic,
and achiral reaction partners. Unprecedented levels of
diastereoselectivity (de >93%) have been
achieved in allylic substitution reactions involving substituted
[(trialkylsilyl)methyl]cyclohexene
derivatives with aldehyde electrophiles. During the course of
these studies, protodesilylation of
allylsilanes has been investigated in detail, and a cocatalyst system
of TiCl4 and Me2AlCl has been
developed that has eliminated silicon loss in the substitution
reactions studied. Lewis acid-promoted
enolization of ester and ketone substrates with chiral centers adjacent
to the carbonyl moiety has
been studied also. It has been shown that this event in our
studies occurs primarily during catalyst
quench. This isomerization is prevented by quenching the catalyst
with a Lewis base, such as
methanol or triethylamine, prior to aqueous workup.
“…[70] Starting from (+)-(2R,5R)-trans-dihydrocarvone, one may obtain (+)-(2R,6R)-trans-γ-irone in 9 stages. After degradation of the side-chain by ozonolysis, with the introduction of a double bond, the resulting enantiomerically pure cyclohexenone is reacted with methyllithium and then directly with pyridinium chlorochromate.…”
“…The strategy is based upon our previous observation of total (or predominant) trans introduction of an electrophile, starting from allylsilanes (X = CH 2 SiMe 3 , Y = CH 2 ), substituted cyclohexenes (X = CH 3 , Y = CH 2 ), or enolates 4b, (X = OLi, Y = O), during the stereoselective synthesis of trans -γ-irone. …”
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