21. Commitments and contingencies Operating lease commitments The Group leases certain of its office properties under non-cancellable operating lease arrangements. The terms of the leases do not contain rent escalation, or contingent rent, renewal, or purchase options. There are no restrictions placed upon the Group by entering into these leases. As of December 31, 2017, the Group had the following operating lease obligations falling due in: Amount US$ 2018 8,077,319 2019 3,551,542 2020 2,541,724 2021 600,001 2022 and thereafter 1,014,391 Total 15,784,977 Capital lease commitments The Group leases corporate aircraft and equipment under non-cancellable capital lease arrangements. The terms of the lease do not contain contingent rent clauses. As of December 31, 2017, the Group had the following minimum lease payments (excluding the portion of the payments representing executory costs, including any profit thereon) falling due in: Amount US$ 2018 6,196,459 2019 5,639,398 2020 5,082,338 2021 2,541,169 2022 and thereafter - Total minimum lease payments 19,459,364 Less interest (3,571,634) Capital lease obligations 15,887,730 Less current maturities of capital lease obligations (4,472,386) Long-term capital lease obligations 11,415,344 Other commitments As of December 31, 2017, the Group had outstanding commitments with respect to non-cancellable construction contracts for real estate development and land use rights purchases as follows: Amount US$ 2018 278,213,253 2019 59,268,253 2020 19,329,097 2021 24,846 2022 and thereafter - Total 356,835,449 Contingencies As at December 31, 2017, the Group provided guarantees of US$ 1,569,802,754 (2016: US$ 1,672,868,791 ), in favor of its customers in respect of mortgage loans granted by banks to such customers for their purchases of the Groups properties where the underlying real estate ownership certificates can only be provided to the banks on a time delay manner due to administrative procedures in the PRC. Pursuant to the terms of the guarantees, upon default in mortgage pa