Have 301 rules before other WP rules:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /subdir/
RewriteRule ^folder1(.*)$ http://example.com/newfolder/ [L,R=301]
RewriteRule ^index\.php$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /subdir/index.php [L]
</IfModule>