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Acetyl migration between the 28- and the 16-hydroxyl groups of primulagenin A.
TetrahedronLettersNo.28, PP. 3213-3214,196S. PergamonPress. Printed in Great Britain.
ACBTYL
A.
MIGRATIOI? BETMESY'NE 28-AND TRR 16-HYDROxTLGROUPS OF IRIMULEO.D. Henaem snd K.G. Lewis Departmentof Organio Chemistry,Universityof Neew &gland, Armidale,N.S.W.,Australia.
(Receive:! in UK 3 April 1968; acceptedfor publication17 April 1968) In an examinationof methodsfor the preparationof partial4
aoetylatedderivatives
of primulageninA (I, B = H) (1) we observedan aoetyl migrationbetween the 28- and the 16=-OHgroups. The migrationwas found to be reversibleand to be catalysedby both aoids and bases and by contactwith cbromatographio alumina. Recent publioationson the rearrangemsntof the triaoetateof theasapogenolD (2) [identicalwith dihydroln5verogeni.n A arxioamelliagenin A (I, R = OH)] on alumina and of dihydropriverogenin A 16-aoetateurxleracid oouiitions(3) prompt us to report our resultsin the simplerowe of primulageninA. In this ease only the two oqgen functionsat Cl6 and C28 are involvedwhereas in the other two examplesthe rearrangementsequencefor the C,6, C22 and C28 funotionsis not obviousand in particular Tschesche&t.
(3) discountthe possibilityof rearrsngemsntbetweenthe m
di-axially
orientedgroups at Ci6 and Ci7. The acetyl migrationwas first observedwhen
x
primulageninA 3,28_diacetate' in benzene solution was allowedto stand in oontaotwith basic alumina. The product isolatedwas the known 3,lwacetate
(4),
while the expeoted(5) 3-monoaaetate was not obtained HO in any quantity. When the reactionwas performedin oold methsnoliopotassiumcarbonatea mixture of 3,16-*and 3,28-diaoetates'* togetherwith 3-monoacetate was obtained. Treatmentof either primulageninA 3,16_diaoetate or 3,28diaoetatewith hydrogenchloride/chloroform gave the ssme mixtureof 3,16- and 3,2&iaoetates oontaminatedwith the products*of aoid oatalysedrearrangement(6). In additionprimulageninA ' The purity of all startingmaterialswas carefullycheckedby t.1.c. The identityof productswas usually determinediy isolationand comparisonwith authenticmaterial. Productsmarked with an asteriskwere identifiedby t.1.c. comparison.
3214
No.28
28-monoacetate
on treatment
with alumina [bee&o or neutral (Woelm)]wse convertedinto a
mixture of 16- and 28-monoacetates and primulagenin4. Similar resultswere obtainedwhen the 28-monoacetate was treatedwith hydrogenohloride/chlorofonn or methanoliapotassiumcarbonate. Internalmigrationof the aoetyl group from the orysen at C28 to that at Ci6 by the usually acceptedmechanismof acetyl migration (7)
obviouslyimpossiblewith ring D in the
ie
chair form. However if ring D changes to a twist boat (from models it appears that ring E accommodatesthis without any substantialalterationand the stabilityof the twist boat should be enhancedas the attachmentof the double bond to $3 of ring D removes some of the usual l,+interactions)the 1-H and C,7-CH20becomesz
group becomes?-equatoriel and the dihedral angle between C,6-C
100'. Brid@ng to form the sir-memberedcyclic intermediatepostulated
for acyl migrationrequiresa closer approachof these two units which, because of the -cis D-E ring fusion, can be providedby conformationalinversionof ris E. Tschesche-et al. (3) observedthe converuionof dihydropriverogenin A lbacetate, under acetonationconditions(sulphuricacid), into the 16,22-isopropylidene-28-acetate, as well as the expectedieomer, and suggestedthat aoetyl migrationoccurredfrom 16+ to 2%
and thence
to 28-OH. A simpler possibilityon our observationswould be direct migrationfrom 16 28-O&
1. O.D. Hensens and K.G. Lewis, unpublishedresults. TetrahedronLetters,j347 T. Nishimura,N. :Fataniand I. KitagIawa, 2. I. Yosiokt?, 3.
R. Tsohesche,P.T. Tjoa and G. Wulff, TetrahedronLetters, 183 (1968).
L. C. Jeger, C. Wieoli and 1,.Ruzicka,Helv. ohim. Acta, a, J.
(1967).
1183 (1946).
'J.F. Johns znd D.X. Jarina, J. OrE. Chem.,28, 2922 (1963).
6. O.D.
Hensenn
and K.J. Lewis, TetrahedronI&t=,
~1olecular 7. R.U. ISemieux, il
IIewYork (1964).
4639 (1965).
Rearrrdgements Pt.11 (ed. P. de 1.14, p.763. _-
Interscience,
to
ACBTYL
A.
MIGRATIOI? BETMESY'NE 28-AND TRR 16-HYDROxTLGROUPS OF IRIMULEO.D. Henaem snd K.G. Lewis Departmentof Organio Chemistry,Universityof Neew &gland, Armidale,N.S.W.,Australia.
(Receive:! in UK 3 April 1968; acceptedfor publication17 April 1968) In an examinationof methodsfor the preparationof partial4
aoetylatedderivatives
of primulageninA (I, B = H) (1) we observedan aoetyl migrationbetween the 28- and the 16=-OHgroups. The migrationwas found to be reversibleand to be catalysedby both aoids and bases and by contactwith cbromatographio alumina. Recent publioationson the rearrangemsntof the triaoetateof theasapogenolD (2) [identicalwith dihydroln5verogeni.n A arxioamelliagenin A (I, R = OH)] on alumina and of dihydropriverogenin A 16-aoetateurxleracid oouiitions(3) prompt us to report our resultsin the simplerowe of primulageninA. In this ease only the two oqgen functionsat Cl6 and C28 are involvedwhereas in the other two examplesthe rearrangementsequencefor the C,6, C22 and C28 funotionsis not obviousand in particular Tschesche&t.
(3) discountthe possibilityof rearrsngemsntbetweenthe m
di-axially
orientedgroups at Ci6 and Ci7. The acetyl migrationwas first observedwhen
x
primulageninA 3,28_diacetate' in benzene solution was allowedto stand in oontaotwith basic alumina. The product isolatedwas the known 3,lwacetate
(4),
while the expeoted(5) 3-monoaaetate was not obtained HO in any quantity. When the reactionwas performedin oold methsnoliopotassiumcarbonatea mixture of 3,16-*and 3,28-diaoetates'* togetherwith 3-monoacetate was obtained. Treatmentof either primulageninA 3,16_diaoetate or 3,28diaoetatewith hydrogenchloride/chloroform gave the ssme mixtureof 3,16- and 3,2&iaoetates oontaminatedwith the products*of aoid oatalysedrearrangement(6). In additionprimulageninA ' The purity of all startingmaterialswas carefullycheckedby t.1.c. The identityof productswas usually determinediy isolationand comparisonwith authenticmaterial. Productsmarked with an asteriskwere identifiedby t.1.c. comparison.
3214
No.28
28-monoacetate
on treatment
with alumina [bee&o or neutral (Woelm)]wse convertedinto a
mixture of 16- and 28-monoacetates and primulagenin4. Similar resultswere obtainedwhen the 28-monoacetate was treatedwith hydrogenohloride/chlorofonn or methanoliapotassiumcarbonate. Internalmigrationof the aoetyl group from the orysen at C28 to that at Ci6 by the usually acceptedmechanismof acetyl migration (7)
obviouslyimpossiblewith ring D in the
ie
chair form. However if ring D changes to a twist boat (from models it appears that ring E accommodatesthis without any substantialalterationand the stabilityof the twist boat should be enhancedas the attachmentof the double bond to $3 of ring D removes some of the usual l,+interactions)the 1-H and C,7-CH20becomesz
group becomes?-equatoriel and the dihedral angle between C,6-C
100'. Brid@ng to form the sir-memberedcyclic intermediatepostulated
for acyl migrationrequiresa closer approachof these two units which, because of the -cis D-E ring fusion, can be providedby conformationalinversionof ris E. Tschesche-et al. (3) observedthe converuionof dihydropriverogenin A lbacetate, under acetonationconditions(sulphuricacid), into the 16,22-isopropylidene-28-acetate, as well as the expectedieomer, and suggestedthat aoetyl migrationoccurredfrom 16+ to 2%
and thence
to 28-OH. A simpler possibilityon our observationswould be direct migrationfrom 16 28-O&
1. O.D. Hensens and K.G. Lewis, unpublishedresults. TetrahedronLetters,j347 T. Nishimura,N. :Fataniand I. KitagIawa, 2. I. Yosiokt?, 3.
R. Tsohesche,P.T. Tjoa and G. Wulff, TetrahedronLetters, 183 (1968).
L. C. Jeger, C. Wieoli and 1,.Ruzicka,Helv. ohim. Acta, a, J.
(1967).
1183 (1946).
'J.F. Johns znd D.X. Jarina, J. OrE. Chem.,28, 2922 (1963).
6. O.D.
Hensenn
and K.J. Lewis, TetrahedronI&t=,
~1olecular 7. R.U. ISemieux, il
IIewYork (1964).
4639 (1965).
Rearrrdgements Pt.11 (ed. P. de 1.14, p.763. _-
Interscience,
to