Stereochemistry of the [2+4] cycloaddition of cyclopentyne

Article abstract of DOI:10.1016/j.tet.2003.11.003

The [2+4] cycloaddition of cyclopentyne with a pair of diastereomeric 1,3-dienes is found to occur with high stereoselectivity. The results support the applicability of the principles of orbital symmetry even in the case of this exceedingly reactive dienophile.

Full text of DOI:10.1016/j.tet.2003.11.003

Tetrahedron 60 (2004) 469–474
Stereochemistry of the [214] cycloaddition of cyclopentyne
b
John C. Gilberta and Duen-Ren Hou
,
*
aDepartment of Chemistry and Biochemistry, The University of Texas at Austin, 1 University Station Stop A5300, Austin, TX 78712, USA
bDepartment of Chemistry, National Central University. No. 300, Jung-Da Road, Jung-Li City, Taoyuan 320, Taiwan, ROC
Received 17 September 2003; revised 4 November 2003; accepted 4 November 2003
Abstract—The [2þ4] cycloaddition of cyclopentyne with a pair of diastereomeric 1,3-dienes is found to occur with high stereoselectivity.
The results support the applicability of the principles of orbital symmetry even in the case of this exceedingly reactive dienophile.
q 2003 Elsevier Ltd. All rights reserved.
1. Introduction
Cyclopentyne (1) undergoes both [2þ2] and [2þ4]
cycloaddition reactions with spiro-1,3-cyclopentadienes 2
(n¼2, 4) as shown in Eq. 1.1 Remarkably, the [2þ2] process
is completely diastereoselective when stereochemically
Scheme 1.
2,5-diphenylisobenzofuran (Eq. 3).6 Although this is a
thermally-allowed reaction and should be stereospecific,4 two
ð1Þ
labeled alkenes are used to trap 1 (Scheme 1).2 This result,
which could be taken as a signal for concert,3 is inconsistent
with the principle of orbital symmetry,4 and we have
recently proposed an alternate mechanism to account
for this outcome.5 The essence of the alternative is that
the [2þ2] cycloadduct forms by stereospecific ring
expansion of a cyclopropylcarbene, itself derived by
[2þ1] cycloaddition of cyclopentyne to the alkene
(Eq. 2).
ð3Þ
factors stimulated our interest in exploring the Diels–
Alder reaction of 1 with a pair of diastereomeric 1,3-
dienes capable of stereorandomization in a non-concerted
process: (1) the unusual nature of the [2þ2] cycloaddi-
tion with cyclopentyne and (2) the predicted diradical
character of the in-plane p-bond of the cycloalkyne;7 the
latter property could foster a stepwise process leading to
stereorandomization (Scheme 2).
ð2Þ
The [2þ4] process has precedent in Wittig’s isolation of
the double Diels–Alder adduct 3 from reaction of 1 with
Fitjer, et al., had previously reported that reaction of
cyclopentyne with 1,3-butadiene afforded the corresponding
[2þ2] cycloadduct exclusively.8 We surmised that this was
because the diene exists predominantly in its s-trans
conformation and dictated our probing the stereochemistry
of the Diels–Alder reaction of cyclopentyne using a system
Keywords: Cyclopentyne; Diels–Alder Reaction; Stereochemistry; Orbital
symmetry.
*
Corresponding author. Tel.: þ1-5124713220; fax: þ1-5124714998;
e-mail address: jgilbert@mail.utexas.edu
0040–4020/$ - see front matter q 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2003.11.003

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J. C. Gilbert, D.-R. Hou / Tetrahedron 60 (2004) 469–474
of the alcohol 8 returned only starting alcohol and polymer.
As previously reported,13 4 was diastereomerically pure.
Turning to our proposed synthesis of the (E,Z)-diene 5,
irradiation (300–350 nm) transformed 4 into a mixture of 5
and the starting diene. The 1H NMR spectrum of the
reaction mixture contained two new vinylic absorptions of
equal intensity and three new resonances ascribable to
methylene groups. No resonances could be detected for
cyclobutene 6. That the isomer formed by irradiation of 4
was 5 was shown by adding a catalytic amount of molecular
iodine to the NMR solution of the two isomers, whereupon
quantitative conversion to 4 occurred, as judged from the 1H
NMR spectrum (Eq. 5)† A photoequilibrium of 4/5¼1:2 was
established upon prolonged irradiation.
Scheme 2.
wherein the diene moiety is locked s-cis. Dienes 4 and 5
(Eq. 4) were selected for the following reasons: (1) the
presence of identical substituents at the termini of the diene
simplifies characterization of the cycloadducts; (2) 1,2-
dialkylidenecyclopentanes generally react faster in Diels–
Alder reactions than do their six-membered ring analogs9
(3) synthesis of diastereomerically pure 4 had been
reported10—because the yields of pericyclic reactions
involving cyclopentyne and unactivated 1,3-dienes are
usually low,1 contamination of the substrate with a minor
amount of a diastereomer could clearly compromise
interpretation of the experimental results; (4) although
synthesis of 5 had not been reported, we anticipated that it
could be prepared by photocyclization of 411 and thermally-
induced conrotatory ring-opening of the resulting cyclo-
butene 6 (Eq. 4).12
ð5Þ
Because attempts to separate 4 and 5 by column
chromatography proved unsuccessful, their kinetic resol-
ution was explored (Eq. 6). Proposed transition states for the
Diels–Alder reaction of 4 and 5 with anhydrides 9 are
shown in Figure 1. Where the phenyl rings of 4 and 5
coplanar with the diene function, steric interactions with the
incoming dienophile should be comparable for both
diastereomers and no kinetic resolution would be expected.
Such coplanarity, however, is unlikely, given the differing
ð4Þ
ð6Þ
2. Synthesis of dienes
It was reported that (E,E)-1,2-dibenzylidenecyclopentane
(4) could be formed from 7, itself the product of
benzylidenation of cyclopentanone, by reaction with either
benzylidenetriphenylphosphorane,10a or with phenyl-
magnesium bromide followed by dehydration
(Scheme 3).10b In our hands, only the Wittig reaction gave
the diene, however. Attempted acid-catalyzed dehydration
steric repulsions expected in the dienes. Distortion from
planarity should be particularly prominent for the phenyl
group on the Z double bond of 5, hindering approach to the
dienophile and thereby increasing the energy of the
transition state for the Diels–Alder reaction. Although
steric interactions involving the dienophile and the phenyl
groups of 4 are likely as well, they should be less important
because of decreased distortion from planarity. This isomer
should thus react faster than 5 with dienophiles 9.
To test our assumptions computationally, the geometries of
the (E,E)- and (E,Z)-dienes were calculated using AM1 and
†
Iodine is known to promote cis–trans isomerization to
thermodynamically more stable alkenes or dienes.14
Scheme 3.

J. C. Gilbert, D.-R. Hou / Tetrahedron 60 (2004) 469–474
471
Figure 1. Possible steric effects in Diels–Alder reaction of 4 and 5 with maleic anhydrides 9.
MM2 methodologies. Table 1 lists the dihedral angle of the
phenyl rings relative to the plane of the diene moiety. Both
computational approaches predict greater rotation out of the
plane for the phenyl ring on the Z double bond of 5. Other
calculated properties of the two dienes that might define
kinetic selectivity in [2þ4] cycloadditions, for example,
bond lengths and HOMO/LUMO energies, are very similar
to each other. Consequently, the success of a kinetic
resolution of 4 and 5 according to Eq. 6 appeared to depend
on differing steric environments of the reacting p-systems.
3. Reactions of cyclopentyne with (E,E)-diene 4 and
(E,Z)-diene 5
Reaction of 4 with a solution derived by adding cyclo-
butanone to diethyl diazomethylphosphonate (DAMP)4 and
sodium hydride (Eq. 7), a combination of reagents known to
produce cyclopentyne,2b afforded two 1:1 adducts of
cyclopentyne and 4 in a ratio of 1:1.6.
Table 1. Calculated dihedral angles (8) between the diene plane and a
phenyl ring
AM1
MM2
ð7Þ
E,E-diene 4
E,Z-diene 5 (E-phenyl)
E,Z-diene 5 (Z-phenyl)
40.9
38.8
64.2
49.4
50.0
59.7
1
Both the H and 13C NMR spectra of the earlier-eluting
fraction were consistent with its being cis-4,8-diphenyl-
1,2,3,4,5,6,7,8-octahydro-s-indacene (10). Definitive proof
of the structure was obtained through X-ray crystallography,
and an ORTEP plot is provided in Figure 2. The cis
relationship of the phenyl rings was defined by the existence
of a two-fold rotation axis of symmetry perpendicular to and
through the center of the cyclohexadiene ring (Fig. 2a). This
stereochemi0cal relationship is seen in Figure 2b. The C2–
In the event, bromomaleic anhydride (9c) afforded the best
selectivity between 4 and 5 of the three anhydrides
evaluated, presumably because the bromo substituent
simultaneously increases the steric hindrance and the
electrophilicity of dienophile 9c; 9b, which also has
increased steric hindrance but decreased electrophilicity
compared to maleic anhydride itself, is inert toward the two
dienes.15 In practice, treating a 2:1 mixture of 4 and 5 with
0.5 equiv. of 9c afforded a 1:73 ratio of the two dienes, as
0
˚
C6 and C2 –C6 bond distance is 1.33 A, typical for a
carbon–carbon double bond, and the C1–C2–C6–C10
dihedral angle is 1.98, showing that the cyclohexadiene ring
is essentially planar.
1
determined by integration of the H NMR spectrum; thus,
(E,Z)-diene 5 was contaminated with slightly over 1% of the
(E,E) diastereomer.
The second isomer formed in the reaction (Eq. 7) had a more
complicated 1H NMR spectrum than 10, possessing among
Figure 2. ORTEP plot of cycloadduct 10: (a) top view and (b) edge view.

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J. C. Gilbert, D.-R. Hou / Tetrahedron 60 (2004) 469–474
other absorptions four vinylic resonances in the range, d
4.4–6.9 ppm. Although the lability of this product pre-
cluded its isolation, the NMR data are consistent with its
being a diastereomer of the triene 12, which could be
derived from thermally promoted conrotatory ring-opening
of the [2þ2] cycloadduct 11 (Eq. 8). That the product was
not the trans isomer of 10 is clear from the NMR analysis,
however. Assuming our tentative structural assignment of
12 is correct and that it arises from 11, the ratio of
[2þ2]:[2þ4] cycloaddition in the reaction of 4 with
cyclopentyne is 1:1.6.‡
diene 5 prevented purification. Diene 5 has a lower
reactivity toward maleic anhydride as compared to 4, and
this meant that a longer reaction time was required to
remove the excess of it; consequently undesired reactions of
the diene, for example, oligomerization and isomerization,
apparently occurred. The similarity in Rf value of the
cycloadduct and its contaminants, their presence in
relatively large amounts, and its low yield made isolation
of the cycloadduct impossible.
These difficulties necessitated using an indirect method to
characterize the presumed [2þ4] cycloadduct. Treating the
reaction mixture with DDQ effected aromatization to 13.
Interpretation of this experimental result, however, was
complicated by the fact that the starting (E,Z)-diene 5 was
contaminated slightly over 1% of the E,E-isomer 4, so that
reaction of this contaminant would produce 10; this in turn
would afford 13 upon treatment of the reaction mixture with
DDQ. Indeed, careful examination by GC of the crude
reaction mixture obtained from the Diels–Alder reaction of
5 with cyclopentyne (Eq. 10) revealed a peak having a
retention time identical to that of 10. Fortunately, the area of
this peak was only 3–4% of that assigned as trans-
cycloadduct 14, whereas the amount of terphenyl 13
produced by oxidation of the reaction mixture was 17–
22% of that of the original 14.{ This proves that the majority
of the 13 formed arose from an isomer different from 10 and
is consistent with this isomer being 14. Thus, although the
structure of 14 has not been proven unambiguously, we
firmly believe it to be the trans isomer.
ð8Þ
Treating 10 with 2,3-dichloro-5,6-dicyano-1,4-benzo-
quinone (DDQ) fostered aromatization to 13 in 75% isolated
yield (Eq. 9).16 This product was characterized spectro-
scopically and provided NMR and MS data entirely
consistent with the structural assignment. Formation of 13
was useful for the subsequent analysis of the products from
reaction of (E,Z)-diene 5 with cyclopentyne (vide infra).
ð9Þ
The result obtained from the reaction of cyclopentyne (1)
with (E,E)-diene 4 (Eq. 7) demonstrates that the [2þ4]
cycloaddition is stereospecific. Such an outcome is entirely
consistent with the transformation being concerted, despite
the unusual nature of the p system of 1 that is involved in
the cycloaddition.
ð10Þ
It might be argued that the formation of 10 in a yield greater
than of percentage (,1%) of diene 4 that contaminants 5
represents diastereomerization during the course of the
[2þ4] cycloaddition of 5. However, knowing that 4 reacts
much faster with dienophiles (vide supra) than does 5, we
ascribe the increase in yield of 10 to this kinetic factor and
believe that 5 reacts in a concerted fashion in the [2þ4]
cycloaddition as does 4.
Extending the study of the stereochemistry of the Diels–
Alder reaction to (E,Z)-diene 5 was complicated by various
factors. Two isomers having the proper value of m/z for a
1:1 adduct of 1 and 5 were formed. One had a GC retention
time identical to that of the product 12 derived from [2þ2]
cycloaddition and was assigned as such (Eq. 10), whereas
the second was tentatively assigned as the [2þ4] cycload-
duct 14. Its retention time was less than that of 10, as shown
by GC analysis of the reaction mixture spiked with authentic
10.§ Unfortunately, the low yield (3–5%) of this isomer and
the presence of impurities generated by treating the reaction
mixture with maleic anhydride to remove excess (E,Z)-
4. Conclusion
The stereochemistry of the Diels–Alder reaction of
cyclopentyne with the (E,E)-diene 4 afforded cycloadduct
12 stereospecifically within experimental error; less than
2% of the diastereomer 14 relative to 10 could have been
detected had it been formed. The result with the (E,Z)-diene
5 is less definitive because of contamination with 4, but a
‘worst-case scenario’ would make the [2þ4] cycloaddition
in this case greater than 95% stereoselective with
retention. Given the enhanced reactivity of 4 relative to 5
‡
The fact that [2þ4] cycloaddition is favored over the [2þ2] analog is
consistent with our previous observations for the reaction of cyclopentyne
with 1,3-cyclopentadienes.1
The shorter retention time of 14 relative to 10 on GC columns is also
§
consistent with the trans structure. The lower yield of oxidation product,
relative to that obtained from 10, may be associated with the known16
slower rate of oxidation of trans-3,6-disubstituted-1,3-cyclohexadienes as
compared to the cis-isomers.
{
These percentages are based on the GC integrations in which two
impurities that are unreactive toward DDQ served as internal standards.

J. C. Gilbert, D.-R. Hou / Tetrahedron 60 (2004) 469–474
473
in the Diels–Alder reaction, however, we believe this
cycloaddition to be stereospecific as well. Consequently, it
can be concluded that the stereocontrol associated with the
principle of orbital symmetry4 still applies, despite the
abnormal characteristics associated with the in-plane
p-bond of cyclopentyne.
5.1.2. Isomerization of (E,Z)-1,2-dibenzylidenecyclo-
pentane (5) to (E,E)-1,2-dibenzylidenecyclopentane (4).
A solution of diene 4 (20 mg) in C6D6 (1 mL) in an NMR
tube was exposed to UV light for 1 h. The 1H NMR
spectrum of the resulting solution indicated formation of a
1:1 mixture of the dienes 4 and 5. Iodine (2 mg) was added
and the resulting dark brown solution was immediately
analyzed by 1H NMR spectroscopy; only resonances of the
diene 4 were observed.
5. Experimental
5.1. General
5.1.3. cis-4,8-Diphenyl-1,2,3,4,5,6,7,8-octahydro-s-inda-
cene (10). Potassium hydride (657 mg, 5.75 mmol, 35% in
mineral oil) in a 50-mL flask was washed with pentane
(3£10 mL). Dry CH2Cl2 (1.5 mL) was added, the slurry was
cooled to 278 8C, and a solution of diethyl (diazomethyl)-
phosphonate (DAMP)17 (760 mg, 4.27 mmol) in CH2Cl2
(3 mL) was transferred into the flask by syringe. The slurry
was stirred for 15 min at 278 8C, cyclobutanone (200 mg,
2.85 mmol) was added, and the reaction mixture was stirred
at 278 8C for an additional 15 min. The cooling bath was
removed, and a warm (50–60 8C) solution of (E,E)-1,2-
dibenzylidenecyclopentane (773 mg, 3.14 mmol) in
benzene (15 mL) was immediately added by syringe. The
solution became dark red, and nitrogen evolution occurred.
The reaction mixture was stirred at rt for 1 h, maleic
anhydride (0.8 g, 8.2 mmol) was added, and the resulting
mixture was stirred for 8 h. The dark-red slurry was poured
into pentane (150 mL), and insoluble material was removed
by vacuum filtration. Concentration of the filtrate followed
by chromatography over silica gel using pentane as eluant
afforded 10 (Rf¼0.45, 12.8 mg, 0.04 mmol, 1.4%) as a
colorless solid, and the presumed [2þ2] cycloadduct (Rf,
7.8 mg, 0.025 mmol, 0.9%) as an amorphous solid.
Analytical GC conditions: initial/temperatures: 120,
250 8C; initial/final time for temperature program: 1,
10 min; ramp rate: 15 8C/min; retention time of 10:
12.9 min.
All reactions were performed under an atmosphere of dry N2
in flamed-dried one-neck flasks equipped for magnetic
stirring. Low-temperature baths of 240 8C were obtained
with an immersion cooler using acetone as the bath liquid.
Ice/water, dry ice/acetone and isooctane/N2(l) were used for
0, 278 and 2107 8C baths, respectively. Commercially
available chemicals were used without further purification
unless noted otherwise. Solvents were dried and distilled
under an inert atmosphere before use. Et2O and THF were
distilled from sodium benzophenone ketyl, CH2Cl2 from
CaH2, and diisopropylamine from KOH. Solutions were
concentrated by rotary evaporation at water aspirator
pressures.
Quantitative GC analyses were obtained with an analytical
gas chromatograph interfaced with an recording integrator
and equipped with a 25 m£0.25 mm AT-1 (100% dimethyl-
polysiloxane) capillary column and a flame-ionization
detector; the carrier gas was helium (1.2 mL/min). GC/MS
analyses were performed using a 12 m£0.22 mm GB-5
(95% dimethyl-, 5% diphenylpolysiloxane) capillary
column with helium as the carrier gas (1.0 mL/min) and
interfaced with an electron impact ion trap detector mass
spectrometer. High-resolution MS analyses were obtained
using the EI mode (70 eV).
Spectral data. 1H NMR: d 1.72 (2H, m), 1.86 (2H, m), 2.09
(8H, m), 3.86 (2H, s), 7.10–7.33 (10H, s); 13C NMR: d 22.2,
33.8, 46.1, 126.2, 128.3 (2C), 136.5, 142.5; HRMS m/z
calcd for C24H25 313.1956, found 313.1948.
1H NMR spectra were obtained at 250 MHz, unless
otherwise noted, and 13C NMR spectra were measured at
125 MHz. All chemical shifts are referenced to the solvent,
which was CDCl3 unless otherwise noted.
X-ray crystallographic analysis for 10: Crystals were grown
as colorless blocks by slow evaporation from a two-phase
solution of pentane–CHCl3. The data crystal was cut from a
larger crystal and had approximate dimensions,
0.35£0.35£0.49 mm3. The data were collected at 290 8C
on a Siemens P3 diffractometer, equipped with a Nicolet
LT-2 low-temperature device and using a graphikte mono-
5.1.1. (E,Z)-1,2-Dibenzylidenecyclopentane (5).
A
solution of E,E-1,2-dibenzylidenecyclopentane11 (1.02 g,
4.14 mmol) in benzene (35 mL) in a 100-mL Pyrex flask
equipped with a septum was exposed to UV light (Hanovia
450-W lamp) for 11 h, after which the solution was filtered
to remove precipitates and concentrated. The residual oil
(0.65 g) was combined with CHCl3 (11 mL), and NaHCO3
(100 mg, 1.2 mmol) in a 50-mL flask. This solution was
cooled to 0 8C, bromomaleic anhydride (233 mg,
1.32 mmol) was added, and the mixture was stirred for
18 h. Filtration, concentration, and chromatography over
silica gel using ethyl acetate–hexane (1:19) as eluant
afforded 5 (Rf¼0.58, 303 mg, 1.23 mmol, 30%) as a
colorless oil.
˚
chromator with Mo Ka radiation (l¼0.71073 A).
5.1.4. Reaction of cyclopentyne with (E,Z)-1,2-dibenzyl-
idenecyclopentane (5). The procedure used for
preparing 10 was used with the following amounts of
reagents: KH (542 mg, 4.74 mmol, 35% in mineral oil),
DAMP (500 mg, 2.80 mmol), cyclobutanone (166 mg,
1
Spectral data. H NMR (500 MHz): d 1.81 (2H, m), 2.53
k
(2H, m), 2.76 (2H, m) 6.48 (1H, s) 6.76 (1H, s); 13C NMR
(C6D6): d 24.2, 33.3, 36.0, 119.0, 123.4, 125.8, 126,.0,
127.7, 128–129 (4C), 138.8, 139.2, 140.7, 144.0; HRMS
m/z calcd for C9H18 246.1409, found 246.1401.
Crystallographic data for the structure in this paper have been deposited
with the Cambridge Crystallographic Data Centre as supplementary
publication numbers CCDC 218855. Copies of the data can be obtained,
free of charge, on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK [fax:þ44-1223-336033 or e-mail: deposit@ccdc.cam.ac.uk].

474
J. C. Gilbert, D.-R. Hou / Tetrahedron 60 (2004) 469–474
3. (a) Sauer, J.; Sustmann, R. Angew. Chem., Int. Ed. Engl. 1980,
19, 779–807, Reviews:. (b) Li, Y.; Houk, K. N. J. Am. Chem.
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4. (a) Woodward, R. B.; Hoffmann, R. The Conservation of
Orbital Symmetry; Verlag-Chemie: Weinheim, 1970.
(b) Fleming, I. Frontier Orbitals and Organic Chemical
Reactions; Wiley: New York, 1976.
2.37 mmol), E,Z-1,2-dibenzylidenecyclopentane (292 mg,
1.19 mmol), and maleic anhydride (0.3 g, 3 mmol). The
filtered pentane solution was concentrated to 10 mL and
passed through a short silica plug before analysis by GC to
afford 47 mg of crude 14. GC conditions were the same as
used for 10, and the retention time of 14 was found to be
12.6 min.
5. (a) Laird, D. W.; Gilbert, J. C. J. Am. Chem. Soc. 2001,
6706–6707. (b) Bachrach, S. M.; Gilbert, J. C.; Laird, D. W.
J. Am. Chem. Soc. 2001, 6708–6709.
5.1.5. 4,8-Diphenyl-1,2,3,5,6,7-hexahydro-s-indacene
(13). A solution of 10 (23 mg, 0.074 mmol) and DDQ
(20 mg, 0.064 mmol) in benzene (5 mL) contained in a
10-mL flask was heated under reflux for 1.5 h, during which
time the solution turned from light yellow to dark black.
Filtration, concentration, and chromatography over silica
gel using ethyl acetate–hexane (1:19) as eluant afforded 13
(Rf (0.49, 15 mg, 0.048 mmol, 75%) as a colorless solid (mp
178–180 8C). Using the same GC conditions as with 10, the
retention time of 13 was 15.9 min.
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Spectra data. 1H NMR: d 1.95 (4H, quintet, J¼7 Hz), 2.81
(8H, t, J¼7 Hz), 7.26–7.44 (10H, m); 13C NMR: d 26.2,
32.7, 126.7, 128.8, 129.6, 133.9, 140.3, 141.1; HRMS m/z
calcd for C24H23 311.1799, found 311.1790.
5.1.6. Dehydrogenation of trans-4,8-diphenyl-1,2,3,
4,5,6,7,8-octahydro-s-indacene (14). The procedure used
for oxidizing 10 was used with the following amounts of
reagents: Crude 14 (47 mg) and DDQ (50 mg, 0.22 mmol).
The solution was heated under reflux for 4.5 h, and the color
of the solution turned from light yellow to dark black. The
resulting solution was vacuum-filtered and concentrated.
The crude product was passed through a silica gel plug using
ethyl acetate–hexane (1:19) as eluant. Analysis by GC
showed that 13 had been formed, and no 14 remained.
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Acknowledgements
We thank the Robert A. Welch Foundation (Grant F-815)
for financial support of this research, Dr. Vincent M. Lynch
for the X-ray crystallographic data, and Professor M. A. Fox
for providing access to a capillary GC.
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