Diphenylphosphinoylethylidene (DPE) acetals: an alternative protective strategy in glycochemistry

Article abstract of DOI:10.1016/j.tetlet.2008.10.111

Diphenylphoshinoylethyne reacts with diols under basic conditions to produce cycloacetalic phosphine oxides. The reaction appears to be general and particularly effective with carbohydrate derivatives. The 2-(diphenylphoshinoyl)ethylidene (DPE) acetals produced are stable in acidic media while they can be cleaved under reductive and/or basic conditions: base-catalyzed transacetalization is a method of choice for their mild and effective deprotection.

Full text of DOI:10.1016/j.tetlet.2008.10.111

Tetrahedron Letters 50 (2009) 101–103
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Diphenylphosphinoylethylidene (DPE) acetals: an alternative protective
strategy in glycochemistry
a
Leonardo Pellizzaro a, Arnaud Tatibouët b, Fabrizio Fabris a, Patrick Rollin b, , Ottorino De Lucchi
*
a Dipartimento di Chimica, Università Ca’ Foscari di Venezia, Dorsoduro 2137, I-30123 Venezia, Italy
b Institut de Chimie Organique et Analytique—UMR 6005, Université d’Orléans, B.P. 6759, F-45067 Orléans, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Diphenylphoshinoylethyne reacts with diols under basic conditions to produce cycloacetalic phosphine
oxides. The reaction appears to be general and particularly effective with carbohydrate derivatives. The
2-(diphenylphoshinoyl)ethylidene (DPE) acetals produced are stable in acidic media while they can be
cleaved under reductive and/or basic conditions: base-catalyzed transacetalization is a method of choice
for their mild and effective deprotection.
Received 19 September 2008
Revised 16 October 2008
Accepted 21 October 2008
Available online 25 October 2008
Ó 2008 Published by Elsevier Ltd.
Keywords:
Carbohydrates
Cyclic acetals
Protective groups
Ethynylphosphine oxide
Selective introduction-expulsion of protective groups still
remains a major concern in polyol and carbohydrate chemistry
strategies, in which cyclic acetals—mainly 1,3-dioxolanes and
1,3-dioxanes—play an impressive role.1,2 In addition to being use-
ful for selective protection of mono-and oligosaccharides, cyclic
acetals can undergo a number of interesting transformations such
as regiospecific oxidative and reductive openings.3 Hence, there is
a persistent demand for new cyclic acetals showing atypical prop-
erties. On those grounds, we have recently developed novel types
of substituted ethylidene acetals: phenylsulfonylethylidene
(PSE)4 and phenylsulfinyl-ethylidene5 acetals, which are easily pre-
pared from miscellaneous diols through base-promoted reaction
with either 1,2-bis-(phenylsulfonyl)ethylene (BPSE) or 1-(phen-
ylsulfinyl)-2-(phenylsulfanyl)ethylene (SOSE): those unusual ace-
tals strongly resist cleavage in acidic media4,6 and display
original properties.7 In the present work, we wish to introduce
diphenylphosphinoylethyne as a new reagent for the protection
of carbohydrates, building up in the lineage of PSE acetals,4 a novel
class of cyclic acetals resistant to cleavage under acidic conditions
and potentially offering additional reactivity features.
be used as precursor to produce diphenylphosphinoylethyne with
equally good yields9 (Scheme 1).
The ability of diphenylphosphinoylethyne to react as Michael
acceptor has been scarcely reported, notably towards amines10a
and halide ions,10b and only a mono-additive process was ob-
served, producing the corresponding vinyl phosphine oxides. How-
ever, when considering the case of related electron-poor alkynes
such as alkyl propynoates11a or arylsulfonylethynes,11b,c it appears
that double nucleophilic additions can take place under forced con-
ditions12 allowing, for example, formation of cyclic acetals and
thioacetals.
Initial exploration was performed on the carbohydrate pyrano-
template 1 which has been currently used as the standard sub-
strate in many of our previous works,4–6 and diverse basic reagents
were tested to activate the hydroxyl groups. As shown in Scheme 2,
the reaction produced the expected chiral 1,3-dioxane 2 in variable
yields, depending on the base involved: DBU showed unreactive,
whereas stronger bases such as tBuOK and LiHMDS only allowed
partial condensation; in concordance with our previous observa-
tions,4,5 optimal conversion into acetal 2 was attained by the use
Diphenylphosphinoylethyne is readily obtainable in good yield
from ethynyltrimethylsilane though a base-promoted nucleophilic
attack to P-chlorodiphenylphosphine, followed by in situ hydrogen
peroxide oxidation, which at the same time removes the trimeth-
ylsilyl moiety.8 Alternatively, inexpensive sodium acetylide can
O
P
1. n-BuLi
2. Ph2PCl
3. H2O2
Ph
Ph
1. Ph2PCl
2. H2O2
H
Na
H
THF
THF
(76%)
TMS
H
(80%)
* Corresponding author.
E-mail address: arnaud.tatibouet@univ-orleans.fr (P. Rollin).
Scheme 1. Preparation of diphenylphosphinoylethyne.
0040-4039/$ - see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.tetlet.2008.10.111

102
L. Pellizzaro et al. / Tetrahedron Letters 50 (2009) 101–103
1. base,n -Bu4NBr,
THF, rt
POPh2
O
OMe
OBn
O
OMe
OBn
O
NaH
THF
HO
HO
HX
YH
n
X
Y
n
O
POPh2
OBn
2
Ph
O
Ph
OBn
2.
H
P
1
9a
, X = Y = O, n = 1 (no reaction)
9b
, X = Y = O, n = 2 (10%)
, X = Y = S, n = 1 (75%)
9c
base (yield): DBU (traces), t-BuOK (38%), LiHMDS (47%), NaH (88%)
POPh2
Scheme 2. Formation of a carbohydrate-based DPE acetal.
POPh2
Et3N
THF
NaH
THF
HO
SH
n
HO
S
n
O
S
n
of sodium hydride in the presence of a catalytic amount of n-
Bu4NBr.13
10a, n = 1 (65%)
10b, n = 2 (70%)
11a, n = 1 (20%)
11b, n = 2 (80%)
The base-promoted protocol established above was applied to a
range of structurally diverse saccharidic diols in order to put up an
introductory evaluation of diphenylphosphinoylethyne’s potential
as protective agent in glycosynthesis. Cyclic DPE acetals of 1,3-
dioxolane- or 1,3-dioxane-type were obtained in 50–75% yields
Scheme 3. Formation of simple DPE acetals.
analogous to those reported for b-phenylsulfonyl- and b-phenyl-
sulfinylethylidene acetals, whose protonated forms are believed
to gain stabilization from a S@O bond.22 However, when submitted
to 90% aqueous trifluoroacetic acid at 60 °C, compound 2 under-
in the
D
-fructopyrano (3),
D-psicopyrano (4), D-glucofurano (5), D-
xylofurano (6)14 and
L
-sorbofurano (7 and 8) series (Fig. 1). As ex-
pected from previous reports, dioxolanic DPE 3, 4 and 5 were
formed with poor diastereoselectivity with regard to the newly
introduced stereogenic centre, whereas dioxanic DPE 2, 6, 7 and
8 bore a strictly equatorially oriented phosphinoyl appendage.
Close functional associations between a phosphine oxide and an
acetal15 or a thioacetal16 moiety have seldom been mentioned in
the literature. It appeared therefore interesting to try and expand
the scope of the reactivity of diphenylphosphinoylethyne towards
simple, flexible diol-type structures; surprisingly and in sharp con-
trast with previous results from our labs,5,7a,11c neither 1,2-ethane-
diol nor 1,3-propanediol could be converted satisfactorily into DPE
acetals. We have observed however, that 1,2-ethanedithiol be-
haved more favourably, affording a 75% yield of the corresponding
dithiolane 9c.17 Oxathiolane 11a18 and oxathiane 11b19 were ob-
tained from 2-mercaptoethanol in low yield and from 3-mercapto-
propanol in good yield, using a modified sequential chemoselective
procedure.20 Initially, primary triethylamine-catalyzed thiol addi-
tion on diphenylphosphinoylethyne produced a Z/E mixture of sul-
fanylvinyl phosphine oxides. This first step proceeded in both cases
with good yields to give intermediates 10a and 10b, which further
underwent NaH-promoted cyclization under standard condi-
tions.13 (see Scheme 3).
TFA/H2O 9:1
60°C, 20 min
O
O
6
OH
OH
Ph2OP
O
12
LiAlH4
60 °C, 20 min
O
HO
HO
O
O
TFA/H2O 9:1
60°C, 20 min
O
OMe
OH
HO
HO
2
1
+
OH
LiAlH4
60°C, 20 min
In order to test the ability of DPE acetals to undergo cleavage
under acidic conditions,21 model acetals 2 and 6 were treated by
a 9:1 (v/v) trifluoroacetic acid/water mixture at room temperature.
The glucopyranoside 2 remained unaffected after 48 h, whereas
the xylofurano compound 6 was selectively 1,2-O-deprotected in
ca 45 min to yield an anomeric mixture of 3,5-O-diphenylphosph-
1
Scheme 4. Behaviour of DPE acetals under deprotection conditions.
inoylethylidene-
D
-xylofuranoses 12 (Scheme 4). These results are
O
O
O
O
O
O
OBn
O
OBn
Ph2OP
O
O
O
O
O
O
O
Ph2OP
BnO
O
Ph2OP
D-fructopyrano 3 (68%)
D-psicopyrano 4 (52%)
BnO
D-glucofurano 5 (66%)
N3
O
O
O
O
O
O
O
O
Ph2OP
Ph2OP
O
O
Ph2OP
O
O
O
O
O
D-xylofurano 6 (75%)
L-sorbofurano 7 (75%)
L-sorbofurano 8 (75%)
Figure 1. Carbohydrate-based DPE acetals.

L. Pellizzaro et al. / Tetrahedron Letters 50 (2009) 101–103
103
11. (a) Ariza, X.; Costa, A. M.; Faja, M.; Pineda, O.; Vilarrasa, J. Org. Lett. 2000, 2,
2809–2811; (b) De Lucchi, O.; Cossu, S. Encyclopedia of Reagents for Organic
Synthesis; John Wiley & Sons, Ltd: USA, 2001; (c) Cossu, S.; De Lucchi, O.; Fabris,
F.; Ballini, R.; Bosica, G. Synthesis 1996, 1481–1484.
Ph2OP
OR
OR
MOH
ROH
2
1
R = Et, 13a
R = Me, 13b
KOH (1M), EtOH 70%
NaOH (1M), EtOH 80%
KOH (1M), MeOH 85%
12. Back, T. G. Tetrahedron 2001, 57, 5263–5301.
13. Typical experimental procedure for the preparation of DPE acetals: To a solution of
diol 1 (749 mg, 2 mmol)4 in dry THF (20 mL) maintained at 0 °C under argon,
NaH (60% dispersion in oil, 200 mg, 2.5 equiv) was introduced. After 15 min
vigorous stirring of the obtained slurry, Bu4NBr (65 mg, 0.05 equiv) was added,
followed by a THF solution of diphenylphosphinoylethyne9 (566 mg, 2.5 mmol
in 10 mL). After stirring for 12 h at rt, the mixture was carefully treated with
brine (15 mL), then extracted with AcOEt (3 Â 10 mL). The combined organic
phase was dried over MgSO4, concentrated in vacuo and the residue was
purified by silica-gel column chromatography (petroleum ether/AcOEt 3:7).
Scheme 5. Retro-Michael transacetalation for deprotection.
went fast (20 min) removal of the 4,6-O-DPE acetal protection to-
gether with partial 2,3-de-O-benzylation: this observation denotes
a marked contrast with the behaviour of PSE acetals and is indica-
tive of a much less efficient stabilization of protonated forms by a
P@O bond.
In contrast, the behaviour of DPE acetals in strongly basic media
compares well with that of PSE acetals. For example, when treated
by LiAlH4 in THF,4a DPE acetals 2 and 6 smoothly release the corre-
sponding diols in nearly quantitative yield (Scheme 4).
Furthermore, cleavage of DPE acetals could also be performed
via base-catalyzed transacetalation using refluxing 1 M ethanol
or methanol solutions of NaOH or KOH. The saccharidic diols were
recovered in high yields, together with 2-(diphenylphosphi-
noyl)acetaldehyde dialkyl acetals 13, the expected side-product
of the deprotection process (Scheme 5).23
In summary, a new class of cyclic acetals bearing a phosphine
oxide in b-position has been introduced in glycochemistry.24 Those
DPE acetals were readily obtained in reasonable yields from diols
and inexpensive diphenylphosphinoylethyne under basic condi-
tions. Although they do not match the high degree of stability of
PSE acetals in acidic media, DPE acetals satisfactorily resist moder-
ate acidic conditions, thanks to a P@O bond stabilization of acti-
vated species. Finally, DPE acetals can undergo efficient cleavage
under basic or reductive conditions. Further chemical reactivity
features of carbohydrate-based DPE acetals are under current
investigation.
Selected data for acetal 2 (syrup, 1.06 g, 88% yield): [
NMR (250 MHz, CDCl3): d 2.68-2.89 (m, 2H, H-8a, H-8b), 3.30 (t, 1H, J3–4 = J4–5
a
]
D +189 (c 1.2, CHCl3); 1
H
=
9.2, H-4), 3.34 (s, 3H, OCH3), 3.39 (t, 1H, J5–6b = Jgem = 10.1, H-6b), 3.43 (dd, 1H,
J1–2 = 3.8, J2–3 = 9.2, H-2), 3.60 (m, 1H, H-5), 3.82 (t, 1H, Jvic = 9.2, H-3), 3.97 (dd,
1H, J5–6a = 4.6, Jgem = 10.1, H-6a), 4.50 (d, 1H, J1–2 = 3.8, H-1), 4.58 (s, 2H,
OCH2Ph), 4.59 and 4.76 (2d, AB system, 2H, Jgem = 12.0, OCH2Ph), 4.96 (bq, 1H,
J7–8 = 5.5, 2JH–P = 10.7, H-7), 7.3 and 7.4 (2m, 16H, H–Ar), 7.6–7.8 (m, 4H, ortho-
1
H–Ar PhPO). 13C NMR (62.5 MHz, CDCl3): d 35.9 (d, JC–P = 70.4, C-8), 55.4
(OMe), 61.9 (C-5), 68.6 (C-6), 73.8 and 75.0 (PhCH2O), 78.5 (C-3), 79.2 (C-2),
81.8 (C-4), 98.0 (C-7), 99.1 (C-1), 127.5–128.7 (CH-Ar), 130.9, 131.0 (2d, 3JC–P
=
4
9.6, CH-meta-PhPO), 131.7, 131.9 (2d, JC–P = 2.7, CH-para-PhPO), 133.0 (d,
1JC–P = 101.9, CIV–PhPO), 133.3 (d, 1JC–P = 102.7, CIV–PhPO), 138.1, 138.8 (2ÃCIV
–
Ar). HRMS calcd for C35H37O7P: 600.2277; found 600.2271.
14. Selected data for DPE acetal 6: [
a
]
+12 (c 2.1, CHCl3); 1H NMR (250 MHz,
D
CDCl3): d 1.27, 1.44 (2s, 6H, Me), 2.61–2.82 (m, 2H, H-7a, H-7b), 3.87 (dd, 1H,
J4–5b = 2.0, Jgem = 13.1, H-5b), 3.95 (m, 1H, H-4), 4.13 (br s, 1H, H-3), 4.15 (bd,
1H, H-5a), 4.19 (bd, H-2), 5.01 (bq, 1H, J6–7 = 5.2, 2JH–P = 10.7, H-6), 5.90 (d, 1H,
J1–2 = 3.7, H-1), 7.38–7.55 (m, 6H, H–Ar), 7.65–7.77 (m, 4H, ortho-H-Ar PhPO).
1
13C NMR (62.5 MHz, CDCl3): d 26.1, 26.6 (iPrd), 36.2 (d, JC–P = 71.6, C-7), 66.2
(C-5), 71.8 (C-4), 78.4 (C-3), 83.4 (C-2), 95.6 (C-1), 105.4 (C-6), 111.8 (CIV–iPrd),
2
3
128.4, 128.5 (2d, JC–P = 12.0, CH-ortho-PhPO), 130.8 (d, JC–P = 9.6, CH-meta-
4
1
PhPO), 131.7, 131.8 (2d, JC–P = 2.6, CH-para-PhPO), 132.8 (d, JC-P = 102.3, CIV
–
PhPO), 133.1 (d, 1JC–P = 102.6, CIV–PhPO). HRMS calcd for C22H25O6P: 416.1389;
found 416.1395.
15. (a) Hansen, K. C.; Wright, C. H.; Aguiar, A. M.; Morrow, C. J.; Turkel, R. M.;
Bhacca, N. S. J. Org. Chem. 1970, 35, 2820–2822; (b) Cates, L. A.; Jones, G. S., Jr.;
Good, D. J.; Tsai, H. Y. L.; Li, V. S.; Caron, N.; Tu, S. C.; Kimball, A. P. J. Med. Chem.
1980, 23, 300–304; (c) Okauchi, T.; Nagamori, H.; Kouno, R.; Ichikawa, J.;
Minami, T. Heterocycles 2000, 52, 1393–1398.
16. Schaumann, E.; Fittkau, S. Bull. Soc. Chim. Belg. 1985, 94, 463–474.
17. Selected data for the syrupy dithiolane 9c: 1H NMR (250 MHz, CDCl3): d 2.96 (dd,
2
2H, Jvic = 7.1, JH–P = 10.2, CH2–PO), 3.05–3.30 (m, 4H, CH2S), 4.83 (dd, 1H,
Jvic = 7.1, Jvic = 13.9, H-2), 7.4–7.6 (m, 6H, H–Ar), 7.70–7.83 (m, 4H, ortho-H–Ar
PhPO). 13C NMR (62.5 MHz, CDCl3): d 31.1 (CH2S), 39.8 (d, 1JC–P = 67.2, CH2–PO),
Acknowledgements
2
2
46.5 (d, JC–P = 3.2, C-2), 128.8 (d, JC–P = 11.8, CH-ortho-PhPO), 131.1 (d,
3JC–P = 9.3, CH–meta-PhPO), 132.1 (d, JC–P = 2.8, CH–para-PhPO), 132.5 (d,
4
We thank Sandrina Silva and Dr. Charlotte Moine (ICOA) for
helpful contribution and comments. This work was co-funded by
MIUR (Rome) within the national PRIN framework. We are also
grateful to EGIDE for support.
1JC–P = 99.2, CIV–PhPO). HRMS calcd for C16H17OPS2: 320.0458; found 320.0452.
18. Selected data for the syrupy oxathiolane 11a: 1H NMR (250 MHz, CDCl3): d 2.80
2
(dd, 1H, Jvic = 7.1, JH–P = 10.1, CHb–PO), 2.98–3.05 (m, 3H, H-4a, H-4b, CHa–
PO), 3.57–3.72 (m, 1H, H-5b), 4.25 (ddd, 1H, H-5a), 5.41 (dd, 1H, Jvic = 5.8,
Jvic = 12.5, H-2), 7.4–7.6 (m, 6H, H–Ar), 7.68–7.83 (m, 4H, ortho-H–Ar PhPO). 13
NMR (62.5 MHz, CDCl3): d 33.4 C-4, 37.9 (d, JC–P = 67.0, CH2–PO), 71.2 (C-5),
C
1
2
3
80.5 (C-2), 128.5 (2d, JC–P = 10.1, CH–ortho-PhPO), 130.9 (2d, JC–P = 9.0, CH–
meta-PhPO), 132.0 (2d, 4JC–P = 2.5, CH–para-PhPO). HRMS calcd for C16H17O2PS:
304.0687; found 304.0691.
References and notes
1. (a) Greene, T. W.; Wuts, P. G. M. Protecting Groups in Organic Synthesis, 3rd ed.;
Wiley & Sons: New York, 1999; (b) Kocienski, P. J. Protecting Groups; Georg
Thieme Verlag: New York, 2004.
2. For reviews in glycochemistry, see: (a) Haines, A. H. Adv. Carbohydr. Chem.
Biochem. 1981, 39, 13–70; (b) Gelas, J. Adv. Carbohydr. Chem. Biochem. 1981, 39,
71–156.
3. (a) Hanessian, S. Methods Carbohydr. Chem. 1972, 6, 183–189; (b) Garegg, P. J.;
Hultberg, H. Carbohydr. Res. 1981, 93, C10–C12; (c) Umemura, K.; Matsuyama,
H.; Kobayashi, M.; Kamigata, N. Bull. Chem. Soc. Jpn. 1989, 62, 3026–3029; (d)
Pohl, N. L.; Kiessling, L. L. Tetrahedron Lett. 1997, 38, 6985–6988.
4. (a) Chéry, F.; Rollin, P.; De Lucchi, O.; Cossu, S. Synthesis 2001, 286–292; (b)
Chéry, F.; Pillard, C.; Tatibouët, A.; De Lucchi, O.; Rollin, P. Tetrahedron 2006, 62,
5141–5151.
19. Selected data for the syrupy oxathiane 11b: 1H NMR (250 MHz, CDCl3): d 1.53 bd,
1H, Jgem = 14.0, H-5b), 1.7–1.9 (m, 1H, H-5a), 2.44–2.67 (m, 2H, H-4b, CHb–PO),
2.76–3.04 (m, 2H, H-4a, CHa–PO), 3.41 (dt, 1H, H-6b), 3.82–3.93 (bd, 1H, H-6a),
2
5.16 dt, Jvic = 8.7, JH–P = 4.0, H-2), 7.32–7.50 (m, 6H, H–Ar), 7.63–7.76 (m, 4H,
ortho-H–Ar PhPO). 13C NMR (62.5 MHz, CDCl3): d 25.2 (C-5), 28.4 (C-4), 37.4 (d,
1JC–P = 68.9, CH2–PO), 69.9 (C-6), 77.3 (d, JC–P = 11.8, C-2), 128.4 (2d, JC–P
=
=
2
2
3
4
11.9, CH–ortho-PhPO), 130.7 (2d, JC–P = 9.7, CH–meta-PhPO), 131.8 (d, JC–P
1 1
2.8, CH–para-PhPO), 133.0 (d, JC–P = 101.6, CIV–PhPO), 133.2 (d, JC–P = 100.7,
CIV–PhPO). HRMS calcd for C17H19O2PS: 318.0843; found 318.0839.
20. Cabianca, E.; Tatibouët, A.; Chéry, F.; Pillard, C.; De Lucchi, O.; Rollin, P.
Tetrahedron Lett. 2003, 44, 5723–5725.
21. Christensen, J. E.; Goodman, L. Carbohydr. Res. 1968, 7, 510–512.
22. An initial mechanistic suggestion given to us by Professor E. Juaristi (Instituto
Politecnico Nacional, Mexico) was later supported by our preliminary ab initio
calculations.
5. Cabianca, E.; Tatibouët, A.; Fabris, F.; De Lucchi, O.; Rollin, P. Tetrahedron Lett.
2005, 46, 1035–1037.
6. (a) Chevalier-du Roizel, B.; Cabianca, E.; Tatibouët, A.; Rollin, P.; Sinaÿ, P.
Tetrahedron 2002, 58, 9579–9583; (b) Uttaro, J. P.; Uttaro, L.; Tatibouët, A.;
Rollin, P.; Mathé, C.; Périgaud, C. Tetrahedron Lett. 2007, 48, 3851–3854.
7. (a) Cabianca, E.; Chéry, F.; Rollin, P.; Tatibouët, A.; De Lucchi, O. Tetrahedron
Lett. 2002, 43, 585–587; (b) Cabianca, E.; Tatibouët, A.; Rollin, P. Pol. J. Chem.
2005, 79, 317–322; (c) Fernandes, A.; Dell’Olmo, M.; Tatibouët, A.; Imberty, A.;
Philouze, C.; Rollin, P. Tetrahedron Lett. 2008, 49, 3484–3488.
8. Corriu, R. J. P.; Guérin, C.; Henner, B. J. L.; Jolivet, A. J. Organomet. Chem. 1997,
530, 39–48.
23. Typical experimental procedure for the base-catalyzed cleavage of DPE acetals: A
sample (0.102 g, 0.17 mmol) of the DPE acetal 2 was added to a 1 M KOH
solution in ethanol (20 mL) and the mixture was refluxed for 12 h, then
concentrated under reduced pressure and partitioned between AcOEt (50 mL)
and brine (20 mL). The organic phase was dried over MgSO4, concentrated and
the residue purified by silica-gel column chromatography (petroleum ether/
AcOEt 1:1, then 3:7). After elution of the diethyl acetal side-product 13a, the
glycoside 1 was recovered (55 mg, 86% yield).
24. The diphenylphosphinoyl moiety was introduced via oxidation of a pyranose-
derived phosphine: (a) Li, C.; Bernet, B.; Vasella, A.; Broger, E. A.; Meili, A.
Carbohydr. Res. 1991, 216, 149–170; (b) Brown, M. A.; Cox, P. J. Organomet.
Chem. 1995, 498, 275–282.
9. Charrier, C.; Chodkiewicz, W.; Cadiot, P. Bull. Soc. Chim. Fr. 1966, 1002–1011.
10. (a) Märkl, G.; Merkl, B. Tetrahedron Lett. 1983, 24, 5865–5868; (b) Huang, X.;
Zhang, C.; Lu, X. Synthesis 1995, 769–771.