26 CFR 301.6323(a)-1: (a)-1 Purchasers, holders of security interests, mechanic's lienors, and judgment lien creditors.

(a) Invalidity of lien without notice. The lien imposed by section 6321 is not valid against any purchaser (as defined in paragraph (f) of § 301.6323(h)—1), holder of a security interest (as defined in paragraph (a) of § 301.6323(h)—1), mechanic's lienor (as defined in paragraph (b) of § 301.6323(h)-1), or judgment lien creditor (as defined in paragraph (g) of § 301.6323(h)-1) until a notice of lien is filed in accordance with § 301.6323(f)-1). Except as provided by section 6323, if a person becomes a purchaser, holder of a security interest, mechanic's lienor, or judgment lien creditor after a notice of lien is filed in accordance with § 301.6323(f)-1, the interest acquired by such person is subject to the lien imposed by section 6321.

(b) Cross references. For provisions relating to the protection afforded a security interest arising after tax lien filing, which interest is covered by a commercial transactions financing agreement, real property construction or improvement financing agreement, or an obligatory disbursement agreement, see §§ 301.6323(c)-1, 301.6323(c)-2, and 301.6323(c)-3, respectively. For provisions relating to the protection afforded to a security interest coming into existence by virtue of disbursements, made before the 46th day after the date of tax lien filing, see § 301.6323(d)-1. For provisions relating to priority afforded to interest and certain other expenses with respect to a lien or security interest having priority over the lien imposed by section 6321, see § 301.6323(e)-1. For provisions relating to certain other interests arising after tax lien filing, see § 301.6323(b)-1.